iflib: emulate counters in netmap mode
When iflib devices are in netmap mode the driver
counters are no longer updated making it look from
userspace tools that traffic has stopped.
Reported by: Franco Fichtner <franco@opnsense.org>
Reviewed by: vmaffione, iflib (erj, gallatin)
Obtained from: OPNsense
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31550
(cherry picked from commit 66fa12d8fb61485780f32f0226e79d3389567496)