6lowpan: Change byte order when storing/accessing to len field

Lenght field should be encoded using big endian byte order, such as intend in the specs.
As it is currently written, the len field would not be decoded properly on an implementation using the correct byte ordering. Hence, it could lead to interroperability issues.

Also, I rewrote the code so that iphc0 argument of lowpan_alloc_new_frame could be removed.

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