selftests/bpf: add missing netfilter return value and ctx access tests

Extend prog_tests with two test cases:

# ./test_progs --allow=verifier_netfilter_retcode
#278/1 verifier_netfilter_retcode/bpf_exit with invalid return code. test1:OK
#278/2 verifier_netfilter_retcode/bpf_exit with valid return code. test2:OK
#278/3 verifier_netfilter_retcode/bpf_exit with valid return code. test3:OK
#278/4 verifier_netfilter_retcode/bpf_exit with invalid return code. test4:OK
#278 verifier_netfilter_retcode:OK

This checks that only accept and drop (0,1) are permitted.

NF_QUEUE could be implemented later if we can guarantee that attachment
of such programs can be rejected if they get attached to a pf/hook that
doesn't support async reinjection.

NF_STOLEN could be implemented via trusted helpers that can guarantee
that the skb will eventually be free'd.

v4: test case for bpf_nf_ctx access checks, requested by Alexei Starovoitov.
v5: also check ctx->{state,skb} can be dereferenced (Alexei).

# ./test_progs --allow=verifier_netfilter_ctx
#281/1 verifier_netfilter_ctx/netfilter invalid context access, size too short:OK
#281/2 verifier_netfilter_ctx/netfilter invalid context access, size too short:OK
#281/3 verifier_netfilter_ctx/netfilter invalid context access, past end of ctx:OK
#281/4 verifier_netfilter_ctx/netfilter invalid context, write:OK
#281/5 verifier_netfilter_ctx/netfilter valid context read and invalid write:OK
#281/6 verifier_netfilter_ctx/netfilter test prog with skb and state read access:OK
#281/7 verifier_netfilter_ctx/netfilter test prog with skb and state read access @unpriv:OK
#281 verifier_netfilter_ctx:OK
Summary: 1/7 PASSED, 0 SKIPPED, 0 FAILED

This checks:
1/2: partial reads of ctx->{skb,state} are rejected
3. read access past sizeof(ctx) is rejected
4. write to ctx content, e.g. 'ctx->skb = NULL;' is rejected
5. ctx->state content cannot be altered
6. ctx->state and ctx->skb can be dereferenced
7. ... same program fails for unpriv (CAP_NET_ADMIN needed).

Link: https://lore.kernel.org/bpf/20230419021152.sjq4gttphzzy6b5f@dhcp-172-26-102-232.dhcp.thefacebook.com/
Link: https://lore.kernel.org/bpf/20230420201655.77kkgi3dh7fesoll@MacBook-Pro-6.local/
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20230421170300.24115-8-fw@strlen.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>