bridge: Enter epoch for bridge_input()/bridge_output()

In stable/12 epoch is not as wide as it is in head. The network stack isn't yet
in epoch when bridge_input()/bridge_output() get called, so rather than assert
this we must enter it ourselves.

This is a direct commit to stable/12.

PR: 248046
Differential Revision: https://reviews.freebsd.org/D25715