selftests: net: properly support IPv6 in GSO GRE test

Explicitly pass -6 to netcat when the test is using IPv6 to prevent
failures.

Also make sure to pass "-N" to netcat to close the socket after EOF on
the client side, otherwise we would always hit the timeout and the test
would fail.

Without this fix applied:

 TEST: GREv6/v4 - copy file w/ TSO                                   [FAIL]
 TEST: GREv6/v4 - copy file w/ GSO                                   [FAIL]
 TEST: GREv6/v6 - copy file w/ TSO                                   [FAIL]
 TEST: GREv6/v6 - copy file w/ GSO                                   [FAIL]

With this fix applied:

 TEST: GREv6/v4 - copy file w/ TSO                                   [ OK ]
 TEST: GREv6/v4 - copy file w/ GSO                                   [ OK ]
 TEST: GREv6/v6 - copy file w/ TSO                                   [ OK ]
 TEST: GREv6/v6 - copy file w/ GSO                                   [ OK ]

Fixes: 025efa0a ("selftests: add simple GSO GRE test")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>