bpf: Add skb->sk, bpf_sk_fullsock and bpf_tcp_sock tests to test_verifer

This patch tests accessing the skb->sk and the new helpers,
bpf_sk_fullsock and bpf_tcp_sock.

The errstr of some existing "reference tracking" tests is changed
with s/bpf_sock/sock/ and s/socket/sock/ where "sock" is from the
verifier's reg_type_str[].

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/verifier/unpriv.c

@@ -365,7 +365,7 @@
 	},
 	.result = REJECT,
 	//.errstr = "same insn cannot be used with different pointers",
-	.errstr = "cannot write into socket",
+	.errstr = "cannot write into sock",
 	.prog_type = BPF_PROG_TYPE_SCHED_CLS,
 },
 {