tools/bpf: adjust rlimit RLIMIT_MEMLOCK for test_dev_cgroup

The default rlimit RLIMIT_MEMLOCK is 64KB. In certain cases,
e.g. in a test machine mimicking our production system, this test may
fail due to unable to charge the required memory for prog load:

  $ ./test_dev_cgroup
  libbpf: load bpf program failed: Operation not permitted
  libbpf: failed to load program 'cgroup/dev'
  libbpf: failed to load object './dev_cgroup.o'
  Failed to load DEV_CGROUP program
  ...

Changing the default rlimit RLIMIT_MEMLOCK to unlimited
makes the test pass.

This patch also fixed a problem where when bpf_prog_load fails,
cleanup_cgroup_environment() should not be called since
setup_cgroup_environment() has not been invoked. Otherwise,
the following confusing message will appear:
  ...
  (/home/yhs/local/linux/tools/testing/selftests/bpf/cgroup_helpers.c:95:
   errno: No such file or directory) Opening Cgroup Procs: /mnt/cgroup.procs
  ...

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