inpcb: Save local group index

inpcb: Save local group index

So that the caller, e.g. UDP protocol, could redistribute the inpcb
accordingly.

We keep the local group sorted by the inpcb local group index in
ascending order.  This eases the multi-process userland application
which uses SO_REUSEPORT sockets and binds process to the owner cpu
of the SO_REUSEPORT socket:
If we didn't sort the local group by the inpcb local group index and
one of the process owning an inpcb in this local group restarted, e.g.
crashed and restarted by watchdog, other processes owning a inpcb in
this local group would have to detect that event, refetch its socket's
owner cpu, and re-bind.