treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

fs/aio.c

@@ -1511,7 +1511,7 @@ static inline void aio_rw_done(struct kiocb *req, ssize_t ret)
 		 * may be already running. Just fail this IO with EINTR.
 		 */
 		ret = -EINTR;
-		/*FALLTHRU*/
+		fallthrough;
 	default:
 		req->ki_complete(req, ret, 0);
 	}