datapath: Don't hold dp_mutex when setting internal devs MTU.

datapath: Don't hold dp_mutex when setting internal devs MTU.

We currently acquire dp_mutex when we are notified that the MTU
of a device attached to the datapath has changed so that we can
set the internal devices to the minimum MTU.  However, it is not
required to hold dp_mutex because we already have RTNL lock and it
causes a deadlock, so don't do it.

Specifically, the issue is that DP mutex is acquired twice: once in
dp_device_event() before calling set_internal_devs_mtu() and then
again in internal_dev_change_mtu() when it is actually being changed
(since the MTU can also be set directly).  Since it's not a recursive
mutex, deadlock.