selftests/bpf: set unlimited RLIMIT_MEMLOCK for test_sock_fields

This is to avoid permission denied error. A lot of systems
may have a much lower number, e.g., 64KB, for RLIMIT_MEMLOCK,
which may not be sufficient for the test to run successfully.

Fixes: e0b27b3f ("bpf: Add test_sock_fields for skb->sk and bpf_tcp_sock")
Signed-off-by: Yonghong Song <yhs@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>