net: contiki: Remember original data length when sending

If the stack is sending network packets by itself, like when
sending router or neighbor solicitation messages, then remember
the original length of the message. This is important because
uIP stack will clear the length of the sent message after it
is sent.
After this change, the lower level drivers can use
the net_buf_datalen() to get the original length of the
buffer. The issue is seen because uIP stack is running in
different fiber than the network device driver.

Change-Id: I6078c5d716bba455a93be9be122e97cd24b8dc0e
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>