selftests/bpf: fix broken build of test_maps

fix multiple build errors and warnings

1.
test_maps.c: In function ‘test_map_rdonly’:
test_maps.c:1051:30: error: ‘BPF_F_RDONLY’ undeclared (first use in this function)
        MAP_SIZE, map_flags | BPF_F_RDONLY);

2.
test_maps.c:1048:6: warning: unused variable ‘i’ [-Wunused-variable]
  int i, fd, key = 0, value = 0;

3.
test_maps.c:1087:2: error: called object is not a function or function pointer
  assert(bpf_map_lookup_elem(fd, &key, &value) == -1 && errno == EPERM);

4.
./bpf_helpers.h:72:11: error: use of undeclared identifier 'BPF_FUNC_getsockopt'
        (void *) BPF_FUNC_getsockopt;

Fixes: e043325b ("bpf: Add tests for eBPF file mode")
Fixes: 6e71b04a ("bpf: Add file mode configuration into bpf maps")
Fixes: cd86d1fd ("bpf: Adding helper function bpf_getsockops")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>