ext4: Fix WARN_ON_ONCE in __ext4_handle_dirty_metadata()

When run fsstress with disk offline/online, I got follow warning:
[   47.395783] ------------[ cut here ]------------
[   47.400554] sd 1:0:0:0: rejecting I/O to offline device
[   47.403327] WARNING: CPU: 3 PID: 2420 at fs/buffer.c:1117 mark_buffer_dirty+0xcd/0xe0
[   47.406093] Modules linked in:
[   47.407322] CPU: 3 PID: 2420 Comm: fsstress Not tainted 3.10.0-862.14.0.6.x86_64 torvalds#136
[   47.409702] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),BIOS ?
-20190727_073836-buildvm-ppc64le-16.ppc.fedoraproject.org-3.fc3104/01/2014
[   47.413142] Call Trace:
[   47.413473] sd 1:0:0:0: rejecting I/O to offline device
[   47.414805]  [<ffffffffa604697f>] dump_stack+0x19/0x1b
[   47.415866]  [<ffffffffa5895988>] __warn+0xd8/0x100
[   47.416970]  [<ffffffffa5895acd>] warn_slowpath_null+0x1d/0x20
[   47.418115]  [<ffffffffa5a5aecd>] mark_buffer_dirty+0xcd/0xe0
[   47.419247]  [<ffffffffa5af6a78>] __ext4_handle_dirty_metadata+0x1a8/0x220
[   47.420408]  [<ffffffffa5af65d3>] ? __ext4_journal_get_write_access+0x53/0x110
[   47.421430]  [<ffffffffa5abeec1>] __ext4_new_inode+0x921/0x1350
[   47.422276]  [<ffffffffa5ad22b9>] ext4_create+0xd9/0x1a0
[   47.423071]  [<ffffffffa5a31763>] vfs_create+0xd3/0x140
[   47.423820]  [<ffffffffa5a33870>] do_last+0x10f0/0x12c0
[   47.424627]  [<ffffffffa5bdc4fc>] ? selinux_file_alloc_security+0x3c/0x60
[   47.425571]  [<ffffffffa5a35877>] path_openat+0xd7/0x640
[   47.426389]  [<ffffffffa605885b>] ? system_call_fastpath+0x22/0x27
[   47.428099]  [<ffffffffa5a1c444>] ? create_object+0x234/0x310
[   47.428922]  [<ffffffffa5a3727d>] do_filp_open+0x4d/0xb0
[   47.429724]  [<ffffffffa5a90b69>] ? files_cgroup_alloc_fd+0x39/0x60
[   47.430623]  [<ffffffffa5a44c35>] ? __alloc_fd+0xe5/0x180
[   47.431361]  [<ffffffffa5a234a7>] do_sys_open+0x137/0x240
[   47.432275]  [<ffffffffa6058795>] ? system_call_after_swapgs+0xa2/0x146
[   47.433214]  [<ffffffffa5a235ce>] SyS_open+0x1e/0x20
[   47.433959]  [<ffffffffa5a23606>] SyS_creat+0x16/0x20
[   47.434675]  [<ffffffffa605885b>] system_call_fastpath+0x22/0x27
[   47.435555]  [<ffffffffa60587a1>] ? system_call_after_swapgs+0xae/0x146
[   47.436468] ---[ end trace f8ed64a9adfa868b ]---

In fact, I got a lot of call trace. All call trace trigger warning when call
__ext4_handle_dirty_metadate.
If there are racing calls to __ext4_handle_dirty_metadate. It's possible for
another writeback result in the buffer being marked with an error after we check
if the buffer is marked as having a write error and the buffer up-to-date flag
is cleared. If that happens mark_buffer_dirty() can end up throwing a WARN_ON_ONCE.
To Fix this warning, lock buffer when call mark_buffer_dirty in function
__ext4_handle_dirty_metadate.

Signed-off-by: Ye Bin <yebin10@huawei.com>

fs/ext4/ext4_jbd2.c

@@ -355,11 +355,13 @@ int __ext4_handle_dirty_metadata(const char *where, unsigned int line,
 					 err);
 		}
 	} else {
+		lock_buffer(bh);
 		set_buffer_uptodate(bh);
 		if (inode)
 			mark_buffer_dirty_inode(bh, inode);
 		else
 			mark_buffer_dirty(bh);
+		unlock_buffer(bh);
 		if (inode && inode_needs_sync(inode)) {
 			sync_dirty_buffer(bh);
 			if (buffer_req(bh) && !buffer_uptodate(bh)) {