udf: Avoid double brelse() in udf_rename()

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

udf: Avoid double brelse() in udf_rename()

syzbot reported a warning like below [1]:

VFS: brelse: Trying to free free buffer
WARNING: CPU: 2 PID: 7301 at fs/buffer.c:1145 __brelse+0x67/0xa0
...
Call Trace:
<TASK>
invalidate_bh_lru+0x99/0x150
smp_call_function_many_cond+0xe2a/0x10c0
? generic_remap_file_range_prep+0x50/0x50
? __brelse+0xa0/0xa0
? __mutex_lock+0x21c/0x12d0
? smp_call_on_cpu+0x250/0x250
? rcu_read_lock_sched_held+0xb/0x60
? lock_release+0x587/0x810
? __brelse+0xa0/0xa0
? generic_remap_file_range_prep+0x50/0x50
on_each_cpu_cond_mask+0x3c/0x80
blkdev_flush_mapping+0x13a/0x2f0
blkdev_put_whole+0xd3/0xf0
blkdev_put+0x222/0x760
deactivate_locked_super+0x96/0x160
deactivate_super+0xda/0x100
cleanup_mnt+0x222/0x3d0
task_work_run+0x149/0x240
? task_work_cancel+0x30/0x30
do_exit+0xb29/0x2a40
? reacquire_held_locks+0x4a0/0x4a0
? do_raw_spin_lock+0x12a/0x2b0
? mm_update_next_owner+0x7c0/0x7c0
? rwlock_bug.part.0+0x90/0x90
? zap_other_threads+0x234/0x2d0
do_group_exit+0xd0/0x2a0
__x64_sys_exit_group+0x3a/0x50
do_syscall_64+0x34/0xb0
entry_SYSCALL_64_after_hwframe+0x63/0xcd

The cause of the issue is that brelse() is called on both ofibh.sbh
and ofibh.ebh by udf_find_entry() when it returns NULL. However,
brelse() is called by udf_rename(), too. So, b_count on buffer_head
becomes unbalanced.

This patch fixes the issue by not calling brelse() by udf_rename()
when udf_find_entry() returns NULL.