ubifs: Fix memory leak of bud->log_hash

Ensure that the allocated bud->log_hash (if any) is freed in all cases
when the bud itself is freed, to fix this leak caught by kmemleak:

 # keyctl add logon foo:bar data @s
 # echo clear > /sys/kernel/debug/kmemleak
 # mount -t ubifs /dev/ubi0_0 mnt -o auth_hash_name=sha256,auth_key=foo:bar
 # echo a > mnt/x
 # umount mnt
 # mount -t ubifs /dev/ubi0_0 mnt -o auth_hash_name=sha256,auth_key=foo:bar
 # umount mnt
 # sleep 5
 # echo scan > /sys/kernel/debug/kmemleak
 # echo scan > /sys/kernel/debug/kmemleak
 # cat /sys/kernel/debug/kmemleak
 unreferenced object 0xff... (size 128):
   comm "mount"
   backtrace:
     __kmalloc
     __ubifs_hash_get_desc+0x5d/0xe0 ubifs
     ubifs_replay_journal
     ubifs_mount
     ...

Fixes: da8ef65f

 ("ubifs: Authenticate replayed journal")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>