bpf: sk_msg, fix sk_msg_md access past end test

Currently, the test to ensure reads past the end of the sk_msg_md
data structure fail is incorrectly expecting success. Fix this
typo and use correct expected error.

Fixes: 945a47d8 ("bpf: sk_msg, add tests for size field")
Reported-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NJohn Fastabend <john.fastabend@gmail.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -1879,7 +1879,7 @@ static struct bpf_test tests[] = {
 				    offsetof(struct sk_msg_md, size) + 4),
 			BPF_EXIT_INSN(),
 		},
-		.errstr = "R0 !read_ok",
+		.errstr = "invalid bpf_context access",
 		.result = REJECT,
 		.prog_type = BPF_PROG_TYPE_SK_MSG,
 	},