selftests: rtnetlink.sh explicitly requires bash.

the script rtnetlink.sh requires a bash-only features (sleep with sub-second
precision). This may cause random test failure if the default shell is not
bash.
Address the above explicitly requiring bash as the script interpreter.

Fixes: 33b01b7b ("selftests: add rtnetlink test script")
Signed-off-by: NPaolo Abeni <pabeni@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/net/rtnetlink.sh

-#!/bin/sh
+#!/bin/bash
 #
 # This test is for checking rtnetlink callpaths, and get as much coverage as possible.
 #