ext4: fix mb_cache_entry's e_refcnt leak in ext4_xattr_block_cache_find()

Syzbot reports a warning as follows:

============================================
WARNING: CPU: 0 PID: 5075 at fs/mbcache.c:419 mb_cache_destroy+0x224/0x290
Modules linked in:
CPU: 0 PID: 5075 Comm: syz-executor199 Not tainted 6.9.0-rc6-gb947cc5b
RIP: 0010:mb_cache_destroy+0x224/0x290 fs/mbcache.c:419
Call Trace:
 <TASK>
 ext4_put_super+0x6d4/0xcd0 fs/ext4/super.c:1375
 generic_shutdown_super+0x136/0x2d0 fs/super.c:641
 kill_block_super+0x44/0x90 fs/super.c:1675
 ext4_kill_sb+0x68/0xa0 fs/ext4/super.c:7327
[...]
============================================

This is because when finding an entry in ext4_xattr_block_cache_find(), if
ext4_sb_bread() returns -ENOMEM, the ce's e_refcnt, which has already grown
in the __entry_find(), won't be put away, and eventually trigger the above
issue in mb_cache_destroy() due to reference count leakage.

So call mb_cache_entry_put() on the -ENOMEM error branch as a quick fix.

Reported-by: syzbot+dd43bd0f7474512edc47@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=dd43bd0f7474512edc47
Fixes: fb265c9c ("ext4: add ext4_sb_bread() to disambiguate ENOMEM cases")
Cc: stable@kernel.org
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240504075526.2254349-2-libaokun@huaweicloud.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/xattr.c

@@ -3117,8 +3117,10 @@ ext4_xattr_block_cache_find(struct inode *inode,
 
 		bh = ext4_sb_bread(inode->i_sb, ce->e_value, REQ_PRIO);
 		if (IS_ERR(bh)) {
-			if (PTR_ERR(bh) == -ENOMEM)
+			if (PTR_ERR(bh) == -ENOMEM) {
+				mb_cache_entry_put(ea_block_cache, ce);
 				return NULL;
+			}
 			bh = NULL;
 			EXT4_ERROR_INODE(inode, "block %lu read error",
 					 (unsigned long)ce->e_value);