dynlink: avoid copying to temp buffer in get_lfs64

while commit 53ac44ff4c0e91536e1e34e8e59e19d2a1196a67 fixed the temp
buffer being undersized, the use of a temp buffer to begin with was a
mistake. instead, compare the requested symbol name in-place and use
the already-null-terminated copy of the name without "64" present in
lfs64_list[] to look up the real symbol.

ldso/dynlink.c

@@ -362,19 +362,14 @@ static struct symdef get_lfs64(const char *name)
 		"pwritev\0readdir\0scandir\0sendfile\0setrlimit\0"
 		"stat\0statfs\0statvfs\0tmpfile\0truncate\0versionsort\0"
 		"__fxstat\0__fxstatat\0__lxstat\0__xstat\0";
-	size_t l;
-	char buf[17];
-	for (l=0; name[l]; l++) {
-		if (l >= sizeof buf) goto nomatch;
-		buf[l] = name[l];
-	}
 	if (!strcmp(name, "readdir64_r"))
 		return find_sym(&ldso, "readdir_r", 1);
-	if (l<2 || name[l-2]!='6' || name[l-1]!='4')
+	size_t l = strnlen(name, 18);
+	if (l<2 || name[l-2]!='6' || name[l-1]!='4' || name[l])
 		goto nomatch;
-	buf[l-=2] = 0;
 	for (p=lfs64_list; *p; p++) {
-		if (!strcmp(buf, p)) return find_sym(&ldso, buf, 1);
+		if (!strncmp(name, p, l-2) && !p[l-2])
+			return find_sym(&ldso, p, 1);
 		while (*p) p++;
 	}
 nomatch: