bpf: Preserve packet pointer class displacement in regsafe()

In the Linux kernel, the following vulnerability has been resolved:

bpf: Preserve packet pointer class displacement in regsafe()

regsafe() maps packet pointer IDs between states and checks that each
current register range is a subset of the corresponding explored
register range. It does not, however, preserve the displacement between
registers that share a packet pointer ID.

This is unsound because packet range is shared by ID. A bounds check on
one class member updates every member, and a later access can consume the
range through another member. Commit 022ac0750883 ("bpf: use reg->var_off
instead of reg->off for pointers") folded the fixed pointer offset into
r64 and removed the old off equality check, so two individually narrower
registers can prune even when their displacement has changed. The
explored path can then license an out-of-bounds packet access on the
pruned path.

Require matching range bases for packet pointers with an ID. Together
with the existing ID mapping, this preserves the displacement between
members of each packet-pointer class without adding per-ID state.
Packet pointers without an ID remain unaffected.