selftest: net: fix socklen_t type mismatch in sctp_collision test

Socket APIs like recvfrom(), accept(), and getsockname() expect socklen_t*
arg, but tests were using int variables. This causes -Wpointer-sign
warnings on platforms where socklen_t is unsigned.

Change the variable type from int to socklen_t to resolve the warning and
ensure type safety across platforms.

warning fixed:

sctp_collision.c:62:70: warning: passing 'int *' to parameter of
type 'socklen_t *' (aka 'unsigned int *') converts between pointers to
integer types with different sign [-Wpointer-sign]
62 | ret = recvfrom(sd, buf, sizeof(buf),
0, (struct sockaddr *)&daddr, &len);
| ^~~~
/usr/include/sys/socket.h:165:27: note: passing argument to
parameter '__addr_len' here
165 | socklen_t *__restrict __addr_len);
| ^

Reviewed-by: Muhammad Usama Anjum <[e-mail]>
Signed-off-by: Ankit Khushwaha <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/netfilter/sctp_collision.c

···
 int main(int argc, char *argv[])
 {
 	struct sockaddr_in saddr = {}, daddr = {};
-	int sd, ret, len = sizeof(daddr);
+	socklen_t len = sizeof(daddr);
 	struct timeval tv = {25, 0};
 	char buf[] = "hello";
+	int sd, ret;
 
 	if (argc != 6 || (strcmp(argv[1], "server") && strcmp(argv[1], "client"))) {
 		printf("%s <server|client> <LOCAL_IP> <LOCAL_PORT> <REMOTE_IP> <REMOTE_PORT>\n",