selftests: mlxsw: bail_on_lldpad before installing the cleanup trap

A number of mlxsw-specific QoS tests use manual QoS DCB management. As
such, they need to make sure lldpad is not running, because it would
override the configuration the test has applied using other tools. To that
end, these selftests invoke the bail_on_lldpad() helper, which terminates
the selftest if th lldpad is running.

Some of these tests however first install the bash exit trap, which invokes
a cleanup() at the test exit. If bail_on_lldpad() has terminated the script
even before the setup part was run, the cleanup part will be very confused.

Therefore make sure bail_on_lldpad() is invoked before the cleanup is
registered.

While there are still edge cases where the user terminates the script
before the setup was fully done, this takes care of a common situation
where the cleanup would be invoked in an inconsistent state.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>