nvme: introduce helper function to get ctrl state

The controller state is typically written by another CPU, so reading it
should ensure no optimizations are taken. This is a repeated pattern in
the driver, so start with adding a convenience function that returns the
controller state with READ_ONCE().
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>

drivers/nvme/host/nvme.h

@@ -387,6 +387,11 @@ struct nvme_ctrl {
 	enum nvme_dctype dctype;
 };
 
+static inline enum nvme_ctrl_state nvme_ctrl_state(struct nvme_ctrl *ctrl)
+{
+	return READ_ONCE(ctrl->state);
+}
+
 enum nvme_iopolicy {
 	NVME_IOPOLICY_NUMA,
 	NVME_IOPOLICY_RR,