The ML2 create_network_db was re-passing in the entire network
with extensions like vlan_transparency present that was causing
issues in the base update function it was calling.
This corrects the behavior by having it only update the MTU, which
is the only thing it was intending to update in the first place.
Change-Id: I723c5c138e0830de98f6024c7635ec65065e9346
Closes-Bug: #1446784
(cherry picked from commit f85de393c4)