selftest: net: fix socklen_t type mismatch in sctp_collision test

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 <usama.anjum@collabora.com>
Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20251028172947.53153-1-ankitkhushwaha.linux@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>