net: ip: 6lo (IPHC) uncompression rework

This is a rework of the 6lo IPHC uncompression.
The uncompression now tries to work in place on the original buffer
instead of allocation a new one. If there is not enough tail-room,
a new buffer is allocated and filled with the IP and maybe UDP header.
The compressed header is pulled from the original buffer and the
buffer is preserved.

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>