zfs_domount: fix double-disown of dataset / double-free of zfsvfs_t

Before this patch, in zfs_domount, if zfs_root or d_make_root fails, we
leave zfsvfs != NULL. This will lead to execution of the error handling `if`
statement at the `out` label, and hence to a call to
dmu_objset_disown and zfsvfs_free.

However, zfs_umount, which we call upon failure of zfs_root and d_make_root
already does dmu_objset_disown and zfsvfs_free.

I suppose this patch rather adds to the brittleness of this part of the code
base, but I don't want to invest more time in this right now.
To add a regression test, we'd need some kind of fault injection facitility for
zfs_root or d_make_root, which doesn't exist right now.
And even then, I think that regression test would be too closely tied to the
implementation.

To repro the double-disown / double-free, do the following:
1. patch zfs_root to always return an error
2. mount a ZFS filesystem

Here's the stack trace you would see then:

  VERIFY3(ds->ds_owner == tag) failed (0000000000000000 == ffff9142361e8000)
  PANIC at dsl_dataset.c:1003:dsl_dataset_disown()
  Showing stack for process 28332
  CPU: 2 PID: 28332 Comm: zpool Tainted: G           O      5.10.103-1.nutanix.el7.x86_64 #1
  Call Trace:
   dump_stack+0x74/0x92
   spl_dumpstack+0x29/0x2b [spl]
   spl_panic+0xd4/0xfc [spl]
   ? find_next_bit+0x14/0x20
   ? __free_pages+0x77/0x80
   ? kfree+0x3a6/0x450
   ? percpu_counter_destroy+0x6a/0x80
   dsl_dataset_disown+0xe9/0x150 [zfs]
   dmu_objset_disown+0xd6/0x150 [zfs]
   zfs_domount+0x17b/0x4b0 [zfs]
   zpl_mount+0x174/0x220 [zfs]
   legacy_get_tree+0x2b/0x50
   vfs_get_tree+0x2a/0xc0
   ? ns_capable+0x10/0x20
   path_mount+0x2fa/0xa70
   do_mount+0x7c/0xa0
   __x64_sys_mount+0x8b/0xe0
   do_syscall_64+0x38/0x50
   entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>

module/os/linux/zfs/zfs_vfsops.c

@@ -1555,14 +1555,18 @@ zfs_domount(struct super_block *sb, zfs_mnt_t *zm, int silent)
 	/* Allocate a root inode for the filesystem. */
 	error = zfs_root(zfsvfs, &root_inode);
 	if (error) {
+		VERIFY3P(zfsvfs->z_os, !=, NULL);
 		(void) zfs_umount(sb);
+		zfsvfs = NULL; /* avoid double-free; first in zfs_unmount */
 		goto out;
 	}
 
 	/* Allocate a root dentry for the filesystem */
 	sb->s_root = d_make_root(root_inode);
 	if (sb->s_root == NULL) {
+		VERIFY3P(zfsvfs->z_os, !=, NULL);
 		(void) zfs_umount(sb);
+		zfsvfs = NULL; /* avoid double-free; first in zfs_unmount */
 		error = SET_ERROR(ENOMEM);
 		goto out;
 	}