selftest/net: fix protocol family to work for IPv4.

use actual protocol family passed by user rather than hardcoded
AF_INTE6 to cerate sockets.
current code is not working for IPv4.
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Signed-off-by: Vaneet Narang <v.narang@samsung.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/tcp_mmap.c

@@ -402,7 +402,7 @@ int main(int argc, char *argv[])
 		exit(1);
 	}
 
-	fd = socket(AF_INET6, SOCK_STREAM, 0);
+	fd = socket(cfg_family, SOCK_STREAM, 0);
 	if (fd == -1) {
 		perror("socket");
 		exit(1);