make getmntent_r discard long lines when it returns error, not seek back

seeking back can be performed by the caller, but if the caller doesn't
expect it, it will result in an infinite loop of failures.

src/linux/mntent.c

@@ -25,7 +25,6 @@ struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int bufle
 		fgets(linebuf, buflen, f);
 		if (feof(f) || ferror(f)) return 0;
 		if (!strchr(linebuf, '\n')) {
-			if (fseeko(f, -(off_t)strlen(linebuf), SEEK_CUR))
 			fscanf(f, "%*[^\n]%*[\n]");
 			errno = ERANGE;
 			return 0;