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/dlm/lock.c

@@ -5817,7 +5817,7 @@ int dlm_user_request(struct dlm_ls *ls, struct dlm_user_args *ua,
 		break;
 	case -EAGAIN:
 		error = 0;
-		/* fall through */
+		fallthrough;
 	default:
 		__put_lkb(ls, lkb);
 		goto out;