This allows efficient filtering at packet ingress on mlx5en.
Note that the packets are filtered (and potentially dropped) *before*
the driver has committed to (re)allocating an mbuf for the
packet. Dropped packets are treated essentially the same as an
error. Nothing is allocated, and the existing buffer is recycled. This
allows us to drop malicious packets at close to line rate with very
little CPU use.
Reviewed by: hselasky, slavash, kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19063
538ff57b75