Btrfs: fix missing mutex_unlock in btrfs_del_dir_entries_in_log()

It is necessary to unlock mutex_lock before it return an error when
btrfs_alloc_path() fails.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>