selftests/bpf: set RLIMIT_MEMLOCK properly for test_libbpf_open.c

Test test_libbpf.sh failed on my development server with failure
  -bash-4.4$ sudo ./test_libbpf.sh
  [0] libbpf: Error in bpf_object__probe_name():Operation not permitted(1).
      Couldn't load basic 'r0 = 0' BPF program.
  test_libbpf: failed at file test_l4lb.o
  selftests: test_libbpf [FAILED]
  -bash-4.4$

The reason is because my machine has 64KB locked memory by default which
is not enough for this program to get locked memory.
Similar to other bpf selftests, let us increase RLIMIT_MEMLOCK
to infinity, which fixed the issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>