selftests: net: use bash to run udpgro_fwd test case

udpgro_fwd.sh contains many bash specific operators ("[[", "local -r"),
but it's using /bin/sh; in some distro /bin/sh is mapped to /bin/dash,
that doesn't support such operators.

Force the test to use /bin/bash explicitly and prevent false positive
test failures.
Signed-off-by: NAndrea Righi <andrea.righi@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/udpgro_fwd.sh

-#!/bin/sh
+#!/bin/bash
 # SPDX-License-Identifier: GPL-2.0
 
 readonly BASE="ns-$(mktemp -u XXXXXX)"