tests/inet-cmsg.c: fix musl libc compilation warnings

tests/inet-cmsg.c: fix musl libc compilation warnings

The types that are used to define msg_namelen member of struct msghdr
and cmsg_len member of struct cmsghdr differ between libcs.
For example, glibc defines them as size_t, while musl libc - as type
socklen_t.  Print these fields using %lu format to avoid issues caused
by libc differences.

* tests/inet-cmsg.c (main): Cast mh.msg_controllen and c->cmsg_len
to unsigned long and print them using %lu format.