Replace ReleaseData function on Packet Structure with ReleasePacket.

This commit allows handling Packets allocated by different methods.
The ReleaseData function pointer in the Packet structure is replaced
with ReleasePacket function pointer, which is then always called to
release the memory associated with a Packet.

Currently, the only usage of ReleaseData is in AF Packet. Previously
ReleaseData was only called when it was not NULL. To implement the
same functionality as before in AF Packet, a new function is defined
in AF Packet to first call the AFP specific ReleaseData function and
then releases the Packet structure.

Three new general functions are defined for releasing packets in the
default case:
  1) PacketFree() - To release a packet alloced with SCMalloc()
  2) PacketPoolReturnPacket() - For packets allocated from the Packet Pool.
  3) PacketFreeOrRelease() - Calls PacketFree() or PacketPoolReturnPacket()
                             based on the PKT_ALLOC flag.

Having these functions removes the need to check the PKT_ALLOC flag
when releasing a packet in most cases, since the ReleasePacket
function encodes how the Packet was allocated. The PKT_ALLOC flag is
still set and is needed when AF Packet releases a packet, since it
replaces the ReleasePacket function pointer with its own function.