netsniff-ng: Clean up HAVE_TPACKET3 #ifdefs

Instead of having #ifdef HAVE_TPACKET3 spread all over the code,
encapsulate the functionality depending on it inside inline functions:
the existing is_tpacket_v3() introduced in commit 5bc19d0b84d0
("netsniff-ng: Only use TPACKET_V3 if HAVE_TPACKET3 is defined") and the
newly introduced get_ring_layout_size() to get the ring layout size
depending on the tpacket version available and the version actually in
use.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>