selftests/bpf: Add reg-invariants test for speculative pointer arithmetic

An unprivileged socket filter does variable pointer arithmetic on a
PTR_TO_MAP_VALUE whose offset collapses to a constant. The Spectre-v1
speculative path used to snapshot the pointer with a const offset and an
unbounded r32, which tripped reg_bounds_sanity_check() on the following
register move.

Mark the test __success_unpriv (the speculative path only runs
unprivileged) and flag it BPF_F_TEST_REG_INVARIANTS so the invariant
violation becomes a hard load failure. The unprivileged run fails without
the verifier fix and passes with it:

  verifier_bounds/spec_ptr_alu_const_offset @unpriv:FAIL   # without fix
  verifier_bounds/spec_ptr_alu_const_offset @unpriv:OK     # with fix

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20260819125840.286434-2-jiayuan.chen@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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