altq: Fix panics on rmc_restart()

rmc_restart() is called from a timer, but can trigger traffic. This
means the curvnet context will not be set.
Use the vnet associated with the interface we're currently processing to
set it. We also have to enter net_epoch here, for the same reason.

Reviewed by: mjg
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31642

(cherry picked from commit 159258afb50ad57f7ed27fe86ded83a7b3a26f90)