CVE-2026-72428: bpf: Fix stack slot index in nospec checks

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

bpf: Fix stack slot index in nospec checks

checkstackwritefixedoff() computes the byte slot for a fixed-offset stack write as -off - 1, and records each written byte in slottype[] with (slot - i) % BPFREGSIZE.

The Spectre v4 sanitization pre-check uses slottype[i] instead. For a 4-byte write at fp-8 after the lower half of fp-8 has been zeroed, the pre-check scans bytes 0..3 and sees STACKZERO while the actual write updates bytes 7..4. That can leave the second half-slot write without nospecresult even though the bytes being overwritten still require sanitization.

Use the same slot index in the sanitization pre-check that the write path uses when updating slottype[].