selftests: rtnetlink.sh explicitly requires bash.

selftests: rtnetlink.sh explicitly requires bash.

BugLink: https://bugs.launchpad.net/bugs/1836654
[ Upstream commit 3c718e677c2b35b449992adc36ecce883c467e98 ]

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: 33b01b7b4f19 ("selftests: add rtnetlink test script")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>