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>

fs/ubifs/ubifs.h

@@ -381,7 +381,7 @@ struct ubifs_gced_idx_leb {
  * @ui_mutex exists for two main reasons. At first it prevents inodes from
  * being written back while UBIFS changing them, being in the middle of an VFS
  * operation. This way UBIFS makes sure the inode fields are consistent. For
- * example, in 'ubifs_rename()' we change 3 inodes simultaneously, and
+ * example, in 'ubifs_rename()' we change 4 inodes simultaneously, and
  * write-back must not write any of them before we have finished.
  *
  * The second reason is budgeting - UBIFS has to budget all operations. If an