Allow changing lagg(4) MTU.

Previously, changing the MTU would require destroying the lagg and
creating a new one. Now it is allowed to change the MTU of
the lagg interface and the MTU of the ports will be set to match.

If any port cannot set the new MTU, all ports are reverted to the original
MTU of the lagg. Additionally, when adding ports, the MTU of a port will be
automatically set to the MTU of the lagg. As always, the MTU of the lagg is
initially determined by the MTU of the first port added. If adding an
interface as a port for some reason fails, that interface is reverted to its
original MTU.

Submitted by: Ryan Moeller <ryan@freqlabs.com>
Reviewed by: mav
Relnotes: Yes
Sponsored by: iXsystems Inc.
Differential Revision: https://reviews.freebsd.org/D17576