net: pkt: Add flag for telling if this is 1st TCP packet

TCP code needs to know whether the pkt is sent the first time
or is it a resent one. This information is used when deciding
if the pkt ref count needs to be increased or not. The packet
does not need to increase ref count when sent first time, as
the ref count is already 1 when the pkt is created. But for the
2nd time the packet is sent, we will need to increase the ref
count in order to avoid buffer leak.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>