Add netmap support for lagg devices

[giuseppe.lettieri_unipi.it]

Summary

Currently, the netmap generic driver reports a successful attachment to lagg devices, but then it is not able to intercept any received packet, since the netmap driver relies on the if_input method which lagg devices do not use.

This patch adds a call to if_input in the lagg input path, when the device has been put in netmap mode, thus enabling full support of netmap mode for these devices.

Test Plan

Tested with WITNESS and INVARIANTS on, using 'pkt-gen -i lagg0 -frx' on a lagg device combining an em and a vtnet device.

[vmaffione]

Support already merged in a different DR.