MFC: if_bridge: change MTU for new members

Rather than reject new bridge members because they have the wrong MTU
change it to match the bridge. If that fails, reject the new interface.

PR: 264883
Different Revision: https://reviews.freebsd.org/D35597

(cherry picked from commit 1865ebfb12ddaf3d0ff1458e6152b3cb1f1bdee8)