Don't pass size_t arguments to setsockopt(SO_SNDBUF/SO_RCVBUF).

These command take an int. The tests work by accident on little-endian,
64-bit systems.

PR: 218919
Tested with: qemu-cheri and CheriBSD built for mips64
Reviewed by: asomers, ngie
Obtained from: CheriBSD
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D10518