[PATCH] nvme: Prevent controller state invalid transition

[Steve Wise]

> 
> Acquiring the nvme_ctrl lock before reading ctrl->state in
> nvme_change_ctrl_state() should prevent a theoretical invalid state
> transition, in the event of two threads racing inside that function.
> 
> I haven't been able to observe this happening with the current code, and
> the current state machine seems to be simple enough to not be
> affected by these invalid transitions, but future modifications could
> make it more likely to happen.
> 
> Signed-off-by: Gabriel Krisman Bertazi <krisman at linux.vnet.ibm.com>

Looks correct.

Reviewed-by: Steve Wise <swise at opengridcomputing.com>