bpf: test_maps add sockmap stress test

Sockmap is a bit different than normal stress tests that can run
in parallel as is. We need to reuse the same socket pool and map
pool to get good stress test cases.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>