UBIFS: increment commit number earlier

Increment the commit number at the beginnig of the commit, instead
of doing this after the commit. This is needed for further
optimizations.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/ubifs.h b/fs/ubifs/ubifs.h
index 6ddd1de..21502b6 100644
--- a/fs/ubifs/ubifs.h
+++ b/fs/ubifs/ubifs.h
@@ -865,7 +865,8 @@
  * @highest_inum: highest used inode number
  * @vfs_gen: VFS inode generation counter
  * @max_sqnum: current global sequence number
- * @cmt_no: commit number (last successfully completed commit)
+ * @cmt_no: commit number of the last successfully completed commit, protected
+ *          by @commit_sem
  * @cnt_lock: protects @highest_inum, @vfs_gen, and @max_sqnum counters
  * @fmt_version: UBIFS on-flash format version
  * @uuid: UUID from super block