ip: reassembly: use correct IP header offset
ip: reassembly: use correct IP header offset

Type: fix
Ticket: VPP-1883
Signed-off-by: Klement Sekera <b8003f27414ca85c37624503b7b4e8f8c829bb4d@cisco.com>
Change-Id: Icf50a645e114fa3b7bd974f8c82f5ceebfcedcd7

chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp,FDio/vpp