6lowpan: fix first fragment (FRAG1) handling

The first fragment, FRAG1, must contain some payload according to the
specs. However, as it is currently written, the first fragment will
remain empty and only contain the 6lowpan headers.

This patch also extracts the transport layer information from the first
fragment. This information is used later on when uncompressing UDP
header.

Thanks to Wolf-Bastian Pöttner for noticing that the offset value was
not properly initialized.

Signed-off-by: Tony Cheneau <tony.cheneau@amnesiak.org>
Signed-off-by: David S. Miller <davem@davemloft.net>