getvfsbyname: prefer sizeof to strlen even for constants

Clang is smart enough to evaluate strlen() of a constant at compile-time.
However, that won't work in the future if we compile libc with
-ffreestanding.

Reported by:	kib
Dissenting:	ngie, cem
Sponsored by:	The FreeBSD Foundation

lib/libc/gen/getvfsbyname.c

@@ -49,10 +49,11 @@ __FBSDID("$FreeBSD$");
 static bool
 are_fusefs(const char *fsname, const char *vfc_name)
 {
-	const char fusefs[] = "fusefs";
-	const char fusefs_dot[] = "fusefs.";
+	const static char fusefs[] = "fusefs";
+	const static char fusefs_dot[] = "fusefs.";
 
-	return (strncmp(fsname, fusefs_dot, strlen(fusefs_dot)) == 0 &&
+
+	return (strncmp(fsname, fusefs_dot, sizeof(fusefs_dot) - 1) == 0 &&
 	    strcmp(fusefs, vfc_name) == 0);
 }