selftests/net: convert gre_gso.sh to run it in unique namespace

Here is the test result after conversion.

 # ./gre_gso.sh
     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 ]

 Tests passed:   4
 Tests failed:   0

Acked-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>