selftests/net: test max_num_members, fanout_args in psock_fanout

Add an additional control test that verifies:
-specifying two different max_num_members values fails
-specifying max_num_members > PACKET_FANOUT_MAX fails

In datapath tests, set max_num_members to PACKET_FANOUT_MAX.

Signed-off-by: Tanner Love <tannerlove@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>