mismatch bi0 and b0 when finalizing ip reassembly.

mismatch bi0 and b0 when finalizing ip reassembly.
bi0 always points to the first buffer in chain but b0 could be any fragment in a ip packet.
It causes a vnet_feature_next function call to the wrong buffer
and crashes sometime when there is >1 feature nodes under ip-unicast.
The fix is simple, makes b0 and bi0 both pointed to the first buffer

Change-Id: I7de36e68fb42b050fa63201abd98aeb6ba2e0cd3
Signed-off-by: Kingwel Xie <kingwel.xie@ericsson.com>