bpf: additional sockmap self tests

Add some more sockmap tests to cover,

 - forwarding to NULL entries
 - more than two maps to test list ops
 - forwarding to different map

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>