net: tcp: Only do ref when resending if we could send the packet

If the packet sending is slow then we must NOT increment the ref
count when re-sending it. This is unlikely but can happen if there
are lot of debug prints etc. extra activities that prevent the driver
to actually send the packet fast enough.

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