Claude review: media: meson: vdec: Fix concurrent STREAMON / STREAMOFF race conditions

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Good idea:** Adding mutex protection around `core->cur_sess` and `sess->status` transitions.

**Issue — `STATUS_INIT` set too early:** The patch sets `sess->status = STATUS_INIT` and `core->cur_sess = sess` inside the mutex *before* any hardware initialization (vififo allocation, poweron, codec start, kthread). If any of these fail, the error unwind correctly resets the state, but there's a window where `sess->status == STATUS_INIT` while the hardware isn't actually initialized. Another call to `vdec_stop_streaming()` during this window could try to stop a partially-initialized session, calling `kthread_stop()` on an uninitialized `recycle_thread`, `vdec_poweroff()` on hardware that was never powered on, etc. The original code set `STATUS_INIT` and `cur_sess` at the end, which was better in that regard (though also racy).

**Issue — `STATUS_INIT` early return bypasses mutex unlock properly:**
```c
if (sess->status == STATUS_INIT) {
    mutex_unlock(&core->lock);
    return 0;
}
```
This is fine structurally but means a second STREAMON on a session that's already STATUS_INIT but not yet STATUS_RUNNING returns success without completing initialization. This might already be the intent of the original code though.

**Issue — stop_streaming sets STATUS_STOPPED before hardware teardown:**
```c
mutex_lock(&core->lock);
old_status = sess->status;
if (core->cur_sess == sess)
    core->cur_sess = NULL;
sess->status = STATUS_STOPPED;
mutex_unlock(&core->lock);
```
Setting `STATUS_STOPPED` and clearing `cur_sess` before actually stopping the hardware means that during the hardware teardown (kthread_stop, vdec_poweroff, etc.), a concurrent `vdec_start_streaming()` could see `STATUS_STOPPED` / `cur_sess == NULL` and attempt to start a new session while the old one is still being torn down. The mutex only protects the state variables, not the actual hardware operations that follow.

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