ubifs: Fix wrong number of inodes locked by ui_mutex in ubifs_inode comment

Since 9ec64962("ubifs: Implement RENAME_EXCHANGE") and
9e0a1fff

("ubifs: Implement RENAME_WHITEOUT") are applied,
ubifs_rename locks and changes 4 ubifs inodes, correct the comment
for ui_mutex in ubifs_inode.

Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>