6lowpan: handling 6lowpan fragmentation via inet_frag api

This patch drops the current way of 6lowpan fragmentation on receiving
side and replace it with a implementation which use the inet_frag api.
The old fragmentation handling has some race conditions and isn't
rfc4944 compatible. Also adding support to match fragments on
destination and source address which is missing in the current
implementation.

This patch add also a lookup function for uncompressed 6LoWPAN header
size. This is needed to estimate the real size of fragmented packet.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: handle return value on lowpan_process_data

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
6lowpan: fix udp nullpointer dereferencing

Sometimes a nullpointer dereferencing occurs because of using a wrong
pointer arithmetic in udp_uncompression.

This patch changes "**(hc06_ptr + 3)" to the right one "*(*hc06_ptr +
3)". Dereferencing like "**(hc06_ptr + 3)" works in a random case only.
6lowpan: fix udp compress ordering

In case ((ntohs(uh->source) & LOWPAN_NHC_UDP_8BIT_MASK) the order of
uncompression is wrong. It's always first source port then destination
port as second.

See:
http://tools.ietf.org/html/rfc6282#section-4.3.3

"Fields carried in-line (in part or in whole) appear in the same order
as they do in the UDP header format"
6lowpan: fix udp byte ordering

The incoming udp header in lowpan_compress_udp_header function is
already in network byte order.

Everytime we read this values for source and destination port we need
to convert this value to host byte order.

In the outcoming header we need to set this value in network byte order
which the upcoming process assumes.
6lowpan: add udp warning for elided checksum

Bit 5 of "UDP LOWPAN_NHC Format" indicate that the checksum can be elided.
The host need to calculate the udp checksum afterwards but this isn't
supported right now.

See:
http://tools.ietf.org/html/rfc6282#section-4.3.3
6lowpan: udp use lowpan_fetch_skb function

Cleanup the lowpan_uncompress_udp_header function to use the
lowpan_fetch_skb function.
6lowpan: udp use subtraction on both conditions

Cleanup code to handle both calculation in the same way.