netdevsim: replace port_list_lock with devlink instance lock

Take advantage of the devlink instance lock for protecting
the port list. This will simplify locking even more once
all devlink callbacks hold the instance lock.

We need to add locking in nsim_dev_port_add_all() which used
to assume higher layer protection when accessing the list.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>