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TCG: probe_access() has inconsistent behavior
Description of problem:
In full-system mode, probe_access() will return NULL when the flag is TLB_MMIO.
accel/tcg/cputlb.c: probe_access_internal()
```
if (unlikely(flags & ~(TLB_WATCHPOINT | TLB_NOTDIRTY | TLB_CHECK_ALIGNED))
|| (access_type != MMU_INST_FETCH && force_mmio)) {
*phost = NULL;
return TLB_MMIO;
}
```
But in linux-user mode, it will return correct address when the flag is TLB_MMIO.
accel/tcg/user-exec.c: probe_access()
```
return size ? g2h(env_cpu(env), addr) : NULL;
```
This will lead to some different behaviors, like cbo.zero in RISC-V.
target/riscv/op_helper.c: helper_cbo_zero()
```
mem = probe_write(env, address, cbozlen, mmu_idx, ra);
if (likely(mem)) {
memset(mem, 0, cbozlen);
} else {
for (int i = 0; i < cbozlen; i++) {
cpu_stb_mmuidx_ra(env, address + i, 0, mmu_idx, ra);
}
}
```
When the current instruction has memory callback by plugin:
Full-system mode uses slow-path(cpu_stb_mmuidx_ra) and inject mem_cbs correctly.
Linux-user mode uses fast-path(memset) and doesn't inject callbacks.
To ensure consistent results, probe_access() should return NULL when the flag is TLB_MMIO in linux-user mode.
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