iflib: Add pfil hooks

As with mlx5en, the idea is to drop unwanted traffic as early
in receive as possible, before mbufs are allocated and anything
is passed up the stack. This can save considerable CPU time
when a machine is under a flooding style DOS attack.

The major change here is to remove the unneeded abstraction where
callers of rxd_frag_to_sd() get back a pointer to the mbuf ring, and
are responsible for NULL'ing that mbuf themselves. Now this happens
directly in rxd_frag_to_sd(), and it returns an mbuf. This allows us
to use the decision (and potentially mbuf) returned by the pfil
hooks. The driver can now recycle mbufs to avoid re-allocation when
packets are dropped.

Reviewed by: marius (shurd and erj also provided feedback)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D19645