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

In the Linux kernel, the following vulnerability has been resolved:

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-gb947cc5bf6d7

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.