selftests: net: refactor reuseport_addr_any test

This patch refactors reuseport_add_any selftest a bit:
- makes it more modular (eliminates several copy/pasted blocks);
- skips DCCP tests if DCCP is not supported

V2: added "Signed-off-by" tag.

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>