libc sctp: fix sctp_getladdrs() for 64-bit BE platforms

When calling getsockopt() with SCTP_GET_LOCAL_ADDR_SIZE, use a
pointer to a 32-bit variable, since this is what the kernel
expects.
While there, do some cleanups.

MFC after: 1 week

(cherry picked from commit dabd8bf9145a0a7255473a482e2ccf31919c3089)