bpf: harden sockmap program attach to ensure correct map type

When attaching a program to sockmap we need to check map type
is correct.

Fixes: 174a79ff ("bpf: sockmap with sk redirect support")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>