lowpan_process_data: return skb instead of using spooky action at a distance

In the case of next header compression (i.e., UDP) we get a new skb.
Alas, the skb change was never communicated back to the caller.

Signed-off-by: Werner Almesberger <werner@almesberger.net>
lowpan_rcv: temporarily remove fragmentation timeout check

Fragmentation timeouts aren't implemented yet, the check's logic was
inverted, and if corrected, it could only cause trouble.

Signed-off-by: Werner Almesberger <werner@almesberger.net>
lowpan_get_frag_info: don't abuse d_size for temporary result

While technically correct, this looks confusing and may give the
compiler an excuse for producing non-optimal code.

Signed-off-by: Werner Almesberger <werner@almesberger.net>