if_bridge: allow MTU changes

if_bridge used to only allow MTU changes if the new MTU matched that of
all member interfaces. This doesn't really make much sense, in that we
really shouldn't be allowed to change the MTU of bridge member in the
first place.

Instead we now change the MTU of all member interfaces. If one fails we
revert all interfaces back to the original MTU.

We do not address the issue where bridge member interface MTUs can be
changed here.

Reviewed by: donner
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31288