net: defragment IP packets

The defragmenting code is enabled by CONFIG_IP_DEFRAG; the code is
useful for TFTP and NFS transfers.  The user can specify the maximum
defragmented payload as CONFIG_NET_MAXDEFRAG (default 16k).
Since NFS has a bigger per-packet overhead than TFTP, the static
reassembly buffer can hold CONFIG_NET_MAXDEFRAG + the NFS overhead.

The packet buffer is used as an array of "hole" structures, acting as
a double-linked list. Each new fragment can split a hole in two,
reduce a hole or fill a hole. No support is there for a fragment
overlapping two diffrent holes (i.e., thre new fragment is across an
already-received fragment).

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>