selftests: udpgso_bench.sh explicitly requires bash

The udpgso_bench.sh script requires several bash-only features. This
may cause random failures if the default shell is not bash.
Address the above explicitly requiring bash as the script interpreter

Fixes: 3a687bef ("selftests: udp gso benchmark")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>