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: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>