selftests/bpf: Add test for reg2btf_ids out of bounds access

selftests/bpf: Add test for reg2btf_ids out of bounds access

This test tries to pass a PTR_TO_BTF_ID_OR_NULL to the release function,
which would trigger a out of bounds access without the fix in commit
45ce4b4f9009 ("bpf: Fix crash due to out of bounds access into reg2btf_ids.")
but after the fix, it should only index using base_type(reg->type),
which should be less than __BPF_REG_TYPE_MAX, and also not permit any
type flags to be set for the reg->type.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220220023138.2224652-1-memxor@gmail.com