selftests/bpf: Test narrow loads with off > 0 for bpf_sock_addr

Add more test cases for context bpf_sock_addr to test narrow loads with
offset > 0 for ctx->user_ip4 field (__u32):
* off=1, size=1;
* off=2, size=1;
* off=3, size=1;
* off=2, size=2.

Signed-off-by: Andrey Ignatov <rdna@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>