selftests/bpf: Test narrow loads for bpf_sock_addr.user_port

Test 1,2,4-byte loads from bpf_sock_addr.user_port in sock_addr
programs.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/e5c734a58cca4041ab30cb5471e644246f8cdb5a.1589420814.git.rdna@fb.com