datapath: Hold dp->mutex when calling new_vport().

datapath: Hold dp->mutex when calling new_vport().

On datapath creation we hold dp_mutex but not dp->mutex when
creating the vport for the datapath device.  However, there are
lockdep checks that validate that we hold dp->mutex during the call
to new_vport().  The lock isn't actually necessary in this case
because no one else can access the datapath but it's good to have
the lock assertions, so this holds dp->mutex while initializing
the datapath.

Found with lockdep.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>