net: tcp: Reset net_pkt creation time if packet is resent

In TCP, we increase the net_pkt refcount in order to resend
it later if we do not receive ACK in time. Because we are not
getting a new net_pkt, the TXTIME statistics would be calculated
incorrectly. So if we re-send, reset the net_pkt creation time.

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