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: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>