There is no mutex protection of these state checking for 'stop'
and 'detach' which can't guarantee there is no another instance
is trying to do same operation.

Consider two instances case:
Instance1: echo stop > /sys/class/remoteproc/remoteproc0/state
Instance2: echo stop > /sys/class/remoteproc/remoteproc0/state

The issue is that the instance2 case may success, Or it
may fail with -EINVAL, which is uncertain.

So move this state checking in rproc_cdev_write() and
state_store() for 'stop', 'detach' operation to
'rproc_shutdown' , 'rproc_detach' function under the mutex
protection.
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1648434012-16655-3-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>