ip: add extended shallow reassembly

ip: add extended shallow reassembly

This patch adds some fixes and improvements:

Fixes bug where save_rewrite_length gets overwritten on reassembly
handoff.

Fixes bug where duplicate fragments could cause a reassembly context
to be lost, because the race losing thread would remove bihash entry
created by winning thread.

Improves tracing by adding more events.

Adds extended shallow reassembly. This is a toggleable option, which if
turned on will cause reassembly to wait for both first and last
fragments to calculate total IP payload length. Furthermore it'll store
a local copy of first fragment and necessary data to retrieve it in
vnet_buffer2. This allows downstream features to access full L3/L4
headers when dealing with fragments.

Type: fix
Change-Id: I81695070533410c5815291dbc65ea71c87e3ae05
Signed-off-by: Klement Sekera <[email protected]>