af-packet: delay setting default-packet-size for af-packet

af-packet: delay setting default-packet-size for af-packet

AF_PACKET needs more information about its configuration before we can
set the default packet size, so on startup, leave unset in suricata.c
if in AF_PACKET mode.

If defrag is enabled, use a default packet size of 9k for tpacket-v2.
This can still lead to truncation events, then the user can increase
their 'default-packet-size'.

Tpacket-v3 does not need an increased packet size as it will handle
any size of packet that is smaller than the configured block size
which now has a default of 128k.

9k for the snap is somewhat arbitrary but is large enough for the
common 9000 jumbo frame plus some extra headers including tpacket
headers.

Ticket: #7458