bpf/cgroup: fix a verification error for a CGROUP_DEVICE type prog

The tools/testing/selftests/bpf test program
test_dev_cgroup fails with the following error
when compiled with llvm 6.0. (I did not try
with earlier versions.)

  libbpf: load bpf program failed: Permission denied
  libbpf: -- BEGIN DUMP LOG ---
  libbpf:
  0: (61) r2 = *(u32 *)(r1 +4)
  1: (b7) r0 = 0
  2: (55) if r2 != 0x1 goto pc+8
   R0=inv0 R1=ctx(id=0,off=0,imm=0) R2=inv1 R10=fp0
  3: (69) r2 = *(u16 *)(r1 +0)
  invalid bpf_context access off=0 size=2
  ...

The culprit is the following statement in dev_cgroup.c:
  short type = ctx->access_type & 0xFFFF;
This code is typical as the ctx->access_type is assigned
as below in kernel/bpf/cgroup.c:
  struct bpf_cgroup_dev_ctx ctx = {
        .access_type = (access << 16) | dev_type,
        .major = major,
        .minor = minor,
  };

The compiler converts it to u16 access while
the verifier cgroup_dev_is_valid_access rejects
any non u32 access.

This patch permits the field access_type to be accessible
with type u16 and u8 as well.

Signed-off-by: Yonghong Song <yhs@fb.com>
Tested-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>