[media] v4l: vb2: Fix race condition in vb2_fop_poll
The vb2_fop_poll() implementation tries to be clever on whether it needs
to lock the queue mutex by checking whether polling might start fileio.
The test requires reading the q->num_buffer field, which is racy if we
don't hold the queue mutex in the first place.
Remove the extra cleverness and just lock the mutex.