selftests/net: Fix broken test case in psock_fanout

The error return falue form sock_fanout_open is -1, not zero.  One test
case was checking for 0 instead of -1.

Tested: Built and tested in clean client.
Signed-off-by: Mike Maloney <maloney@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>