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: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>