6lowpan: lowpan_uncompress_addr with address_mode

This patch drops the pre and postcount calculation from the
lowpan_uncompress_addr function.We use instead a switch/case
over address_mode value. The original implementation has several
bugs in this function and it was hard to decrypt how it works.
To make it maintainable and fix these bugs this patch basically
reimplements lowpan_uncompress_addr from scratch.

A list of bugs we found in the current implementation:

1) Properly support uncompression of short-address based IPv6 addresses
   (instead of basically copying garbage)

2) Fix use and uncompression of long-addresses based IPv6 addresses

3) Add missing ff:fe00 in the case of SAM/DAM = 2 and M = 0

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