cxgbe(4): knobs to drop various kinds of undesirable frames on ingress.

These kind of drops come for free in the sense that they do not use the
filter TCAM or any other resource that wouldn't normally be used during
rx. Frames dropped by the hardware get counted in the MAC's rx stats
but are not delivered to the driver.

hw.cxgbe.attack_filter
Set to 1 to enable the "attack filter". Default is 0. The attack
filter will drop an incoming frame if any of these conditions is true:
src ip/ip6 == dst ip/ip6; tcp and src/dst ip is not unicast; src/dst ip
is loopback (127.x.y.z); src ip6 is not unicast; src/dst ip6 is loopback
(::1/128) or unspecified (::/128); tcp and src/dst ip6 is mcast
(ff00::/8).

hw.cxgbe.drop_ip_fragments
Set to 1 to drop all incoming IP fragments. Default is 0. Note that
this drops valid frames.

hw.cxgbe.drop_pkts_with_l2_errors
Set to 1 to drop incoming frames with Layer 2 length or checksum errors.
Default is 1.

hw.cxgbe.drop_pkts_with_l3_errors
Set to 1 to drop incoming frames with IP version, length, or checksum
errors. Default is 0.

hw.cxgbe.drop_pkts_with_l4_errors
Set to 1 to drop incoming frames with Layer 4 length, checksum, or other
errors. Default is 0.

MFC after: 2 weeks
Sponsored by: Chelsio Communications