mkswap, readahead: stop using fdlength, it is reported to be unreliable

miscutils/readahead.c

@@ -22,9 +22,16 @@ int readahead_main(int argc, char **argv)
 	while (*++argv) {
 		int fd = open_or_warn(*argv, O_RDONLY);
 		if (fd >= 0) {
-			int r = readahead(fd, 0, fdlength(fd));
+			off_t len;
+			int r;
+
+			/* fdlength was reported to be unreliable - use seek */
+			len = xlseek(fd, 0, SEEK_END);
+			xlseek(fd, 0, SEEK_SET);
+			r = readahead(fd, 0, len);
 			close(fd);
-			if (r >= 0) continue;
+			if (r >= 0)
+				continue;
 		}
 		retval = EXIT_FAILURE;
 	}