MFC :r340436

MFC :r340436

vtnet: fix netmap support

netmap(4) support for vtnet(4) was incomplete and had multiple bugs.
This commit fixes those bugs to bring netmap on vtnet in a functional state.

Changelist:
  - handle errors returned by virtqueue_enqueue() properly (they were
    previously ignored)
  - make sure netmap XOR rest of the kernel access each virtqueue.
  - compute the number of netmap slots for TX and RX separately, according to
    whether indirect descriptors are used or not for a given virtqueue.
  - make sure sglist are freed according to their type (mbufs or netmap
    buffers)
  - add support for mulitiqueue and netmap host (aka sw) rings.
  - intercept VQ interrupts directly instead of intercepting them in txq_eof
    and rxq_eof. This simplifies the code and makes it easier to make sure
    taskqueues are not running for a VQ while it is in netmap mode.
  - implement vntet_netmap_config() to cope with changes in the number of queues.

Sponsored by:	Sunny Valley Networks
Differential Revision:	https://reviews.freebsd.org/D17916
Approved by: re (gjb)