netdevsim: replace vfs_lock with devlink instance lock

Similarly to the previous commit, use the devlink instance
lock and let it replace the vfs_lock.

nsim_esw_legacy_enable() was locked by both port lock and
vfs lock so one set of lock/unlocks goes away.

netdevsim's .eswitch_mode_set callback is now ready for
the callback to take the instance lock.

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