selftests/bpf: Fix endianness issue in test_sockopt_sk

getsetsockopt() calls getsockopt() with optlen == 1, but then checks
the resulting int. It is ok on little endian, but not on big endian.

Fix by checking char instead.

Fixes: 8a027dc0 ("selftests/bpf: add sockopt test that exercises sk helpers")
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/20200915113928.3768496-1-iii@linux.ibm.com