6lowpan: add uncompress header size function

This patch add a lookup function for uncompressed 6LoWPAN header
size. This is needed to estimate the real size after uncompress the
6LoWPAN header.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>