nfp: flower: restore RTNL locking around representor updates

nfp: flower: restore RTNL locking around representor updates

When we moved to updating representors from a workqueue grabbing
the RTNL somehow got lost in the process.  Restore it, and make
sure RCU lock is not held while we are grabbing the RTNL.  RCU
protects the representor table, so since we will be under RTNL
we can drop RCU lock as soon as we find the netdev pointer.
RTNL is needed for the dev_set_mtu() call.

Fixes: 2dff19622421 ("nfp: process MTU updates from firmware flower app")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>