[DCCP] ackvec: infrastructure for sending more than one ackvec per packet

[DCCP] ackvec: infrastructure for sending more than one ackvec per packet

Commiter note:

This was split from Andrea's original patch, in the process I changed the type
of the ackvec index fields to u16 instead of to int and haven't folded
dccp_ackvec_parse with dccp_ackvec_check_rcv_ackno.

Next patch will actually do the insertion of more than one ackvec per packet,
using, initially, up to a max of 2 ackvecs as per Andrea's original patch, then
I'll work on support for larger ackvecs, be it using a sysctl or using
setsockopt.

Signed-off-by: Andrea Bittau <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>