Make it possible to set a larger MTU by attempting to set MTUs on all
trunk ports first. If that succeeds, and we're inside our own bounds,
so be it.

Still not ideal -- adding a port after changing an MTU doesn't change
port's MTU, but a step in the right direction.

PR: kern/95417
Submitted by: Vladimir Ivanov <wawa AT yandex-team DOT ru>
MFC after: 3 days

I've slightly edited a patch to make the conditional logic positive
and remove (what I think was) a redundant ng_fec_init() call.