net: pkt: Have separate create time for net_pkt

This value is used to measure the RX/TX statistics. The previous
use of the timestamp field did not work in RX path as the timestamp
value could be overwritten by the driver if gPTP timestamping
is enabled. So to fix the RX statistics, use a separate field
for the create time.

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