net: devlink: remove devlink_mutex

All accesses to devlink structure from userspace and drivers are locked
with devlink->lock instance mutex. Also, devlinks xa_array iteration is
taken care of by iteration helpers taking devlink reference.

Therefore, remove devlink_mutex as it is no longer needed.
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>