net: tcp: Fix the issues with 6lo

When sending TCP data using 802.15.4 or Bluetooth technologies,
the 6lo code modified the original IPv6 header. This caused
issue when acknowledgment was waited to the sent packet as
the code could never match the sequence and ack numbers in
TCP header.

This commit changes this and the packet is cloned when sending
it so the 6lo code will modify a copy of the data and will not
touch the original packet.

JIRA: ZEP-1719

Change-Id: Iae51f35d5b5ada0d2543b58a29abbf10f146777e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>