selftests/bpf: Fix endianness issue in sk_assign

server_map's value size is 8, but the test tries to put an int there.
This sort of works on x86 (unless followed by non-0), but hard fails on
s390.

Fix by using __s64 instead of int.

Fixes: 2d7824ff ("selftests: bpf: Add test for sk_assign")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200915113815.3768217-1-iii@linux.ibm.com