interface: fix the incorrect sizes/offsets in the tso segmentation
interface: fix the incorrect sizes/offsets in the tso segmentation

The copying of the first segment copied too small amount of data.
The copying of the second and subsequent segments used the wrong
data offset - for the case of GSO enabled it starts from 12 bytes
earlier.

Change-Id: I3adc532c175babc1ca1e121c7e12e6cafbdb9974
Type: fix
Ticket: VPP-1700
Signed-off-by: Andrew Yourtchenko <eb6233fe2fbd585844f4fb8426b53b186ab379b1@gmail.com>

FDio/vpp,vpp-dev/vpp,FDio/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp,chrisy/vpp