af-packet: simplify socket handling in tpacket v3

af-packet: simplify socket handling in tpacket v3

Tpacket v3 only supports workers mode, which means the packet that would
reference a socket won't leave the thread. Therefore keeping a ref count
on the socket is not needed.

This patch removes the per packet reference count increment. The decrement
was missing, so this fixes the ref cnt handling so that after a iface up/
down capture can recover.

It should also lead to a minor performance increase as we avoid a round
of atomic operations per packet.

Bug: #4804.
Bug: #4801.