MFC r341535:
ipoib: correct setting MTU from inside ipoib(4).

It is not enough to set ifnet->if_mtu to change the interface MTU.
System saves the MTU for route in the radix tree, and route cache keeps
the interface MTU as well. Since addition of the multicast group causes
recalculation of MTU, even bringing the interface up changes MTU from
4042 to 1500, which makes the system configuration inconsistent. Worse,
ip_output() prefers route MTU over interface MTU, so large packets are
not fragmented and dropped on floor.

Fix it for ipoib(4) using the same approach (or hack) as was applied
for it_tun/if_tap in r339012. Thanks to bz@ for giving the hint.

Sponsored by: Mellanox Technologies