ubifs: Add comment on c->commit_sem

Every single time I come across that code, I get confused
because it looks like a possible dead lock.
Help myself by adding a comment.

Signed-off-by: Richard Weinberger <richard@nod.at>

fs/ubifs/journal.c

@@ -317,6 +317,7 @@ again:
 	down_read(&c->commit_sem);
 	err = reserve_space(c, jhead, len);
 	if (!err)
+		/* c->commit_sem will get released via finish_reservation(). */
 		return 0;
 	up_read(&c->commit_sem);