cifs: fix double free race when mount fails in cifs_get_root()

When cifs_get_root() fails during cifs_smb3_do_mount() we call
deactivate_locked_super() which eventually will call delayed_free() which
will free the context.
In this situation we should not proceed to enter the out: section in
cifs_smb3_do_mount() and free the same resources a second time.

[Thu Feb 10 12:59:06 2022] BUG: KASAN: use-after-free in rcu_cblist_dequeue+0x32/0x60
[Thu Feb 10 12:59:06 2022] Read of size 8 at addr ffff888364f4d110 by task swapper/1/0

[Thu Feb 10 12:59:06 2022] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G           OE     5.17.0-rc3+ Nuvoton-Israel#4
[Thu Feb 10 12:59:06 2022] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.0 12/17/2019
[Thu Feb 10 12:59:06 2022] Call Trace:
[Thu Feb 10 12:59:06 2022]  <IRQ>
[Thu Feb 10 12:59:06 2022]  dump_stack_lvl+0x5d/0x78
[Thu Feb 10 12:59:06 2022]  print_address_description.constprop.0+0x24/0x150
[Thu Feb 10 12:59:06 2022]  ? rcu_cblist_dequeue+0x32/0x60
[Thu Feb 10 12:59:06 2022]  kasan_report.cold+0x7d/0x117
[Thu Feb 10 12:59:06 2022]  ? rcu_cblist_dequeue+0x32/0x60
[Thu Feb 10 12:59:06 2022]  __asan_load8+0x86/0xa0
[Thu Feb 10 12:59:06 2022]  rcu_cblist_dequeue+0x32/0x60
[Thu Feb 10 12:59:06 2022]  rcu_core+0x547/0xca0
[Thu Feb 10 12:59:06 2022]  ? call_rcu+0x3c0/0x3c0
[Thu Feb 10 12:59:06 2022]  ? __this_cpu_preempt_check+0x13/0x20
[Thu Feb 10 12:59:06 2022]  ? lock_is_held_type+0xea/0x140
[Thu Feb 10 12:59:06 2022]  rcu_core_si+0xe/0x10
[Thu Feb 10 12:59:06 2022]  __do_softirq+0x1d4/0x67b
[Thu Feb 10 12:59:06 2022]  __irq_exit_rcu+0x100/0x150
[Thu Feb 10 12:59:06 2022]  irq_exit_rcu+0xe/0x30
[Thu Feb 10 12:59:06 2022]  sysvec_hyperv_stimer0+0x9d/0xc0
...
[Thu Feb 10 12:59:07 2022] Freed by task 58179:
[Thu Feb 10 12:59:07 2022]  kasan_save_stack+0x26/0x50
[Thu Feb 10 12:59:07 2022]  kasan_set_track+0x25/0x30
[Thu Feb 10 12:59:07 2022]  kasan_set_free_info+0x24/0x40
[Thu Feb 10 12:59:07 2022]  ____kasan_slab_free+0x137/0x170
[Thu Feb 10 12:59:07 2022]  __kasan_slab_free+0x12/0x20
[Thu Feb 10 12:59:07 2022]  slab_free_freelist_hook+0xb3/0x1d0
[Thu Feb 10 12:59:07 2022]  kfree+0xcd/0x520
[Thu Feb 10 12:59:07 2022]  cifs_smb3_do_mount+0x149/0xbe0 [cifs]
[Thu Feb 10 12:59:07 2022]  smb3_get_tree+0x1a0/0x2e0 [cifs]
[Thu Feb 10 12:59:07 2022]  vfs_get_tree+0x52/0x140
[Thu Feb 10 12:59:07 2022]  path_mount+0x635/0x10c0
[Thu Feb 10 12:59:07 2022]  __x64_sys_mount+0x1bf/0x210
[Thu Feb 10 12:59:07 2022]  do_syscall_64+0x5c/0xc0
[Thu Feb 10 12:59:07 2022]  entry_SYSCALL_64_after_hwframe+0x44/0xae

[Thu Feb 10 12:59:07 2022] Last potentially related work creation:
[Thu Feb 10 12:59:07 2022]  kasan_save_stack+0x26/0x50
[Thu Feb 10 12:59:07 2022]  __kasan_record_aux_stack+0xb6/0xc0
[Thu Feb 10 12:59:07 2022]  kasan_record_aux_stack_noalloc+0xb/0x10
[Thu Feb 10 12:59:07 2022]  call_rcu+0x76/0x3c0
[Thu Feb 10 12:59:07 2022]  cifs_umount+0xce/0xe0 [cifs]
[Thu Feb 10 12:59:07 2022]  cifs_kill_sb+0xc8/0xe0 [cifs]
[Thu Feb 10 12:59:07 2022]  deactivate_locked_super+0x5d/0xd0
[Thu Feb 10 12:59:07 2022]  cifs_smb3_do_mount+0xab9/0xbe0 [cifs]
[Thu Feb 10 12:59:07 2022]  smb3_get_tree+0x1a0/0x2e0 [cifs]
[Thu Feb 10 12:59:07 2022]  vfs_get_tree+0x52/0x140
[Thu Feb 10 12:59:07 2022]  path_mount+0x635/0x10c0
[Thu Feb 10 12:59:07 2022]  __x64_sys_mount+0x1bf/0x210
[Thu Feb 10 12:59:07 2022]  do_syscall_64+0x5c/0xc0
[Thu Feb 10 12:59:07 2022]  entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/cifsfs.c

@@ -919,6 +919,7 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 
 out_super:
 	deactivate_locked_super(sb);
+	return root;
 out:
 	if (cifs_sb) {
 		kfree(cifs_sb->prepath);