bpf: Fix constant blinding for PROBE_MEM32 stores

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

bpf: Fix constant blinding for PROBE_MEM32 stores

BPF_ST | BPF_PROBE_MEM32 immediate stores are not handled by

bpf_jit_blind_insn(), allowing user-controlled 32-bit immediates to

survive unblinded into JIT-compiled native code when bpf_jit_harden >= 1.

The root cause is that convert_ctx_accesses() rewrites BPF_ST|BPF_MEM

to BPF_ST|BPF_PROBE_MEM32 for arena pointer stores during verification,

before bpf_jit_blind_constants() runs during JIT compilation. The

blinding switch only matches BPF_ST|BPF_MEM (mode 0x60), not

BPF_ST|BPF_PROBE_MEM32 (mode 0xa0). The instruction falls through

unblinded.

Add BPF_ST|BPF_PROBE_MEM32 cases to bpf_jit_blind_insn() alongside the

existing BPF_ST|BPF_MEM cases. The blinding transformation is identical:

load the blinded immediate into BPF_REG_AX via mov+xor, then convert

the immediate store to a register store (BPF_STX).

The rewritten STX instruction must preserve the BPF_PROBE_MEM32 mode so

the architecture JIT emits the correct arena addressing (R12-based on

x86-64). Cannot use the BPF_STX_MEM() macro here because it hardcodes

BPF_MEM mode; construct the instruction directly instead.