nvme: Move transports to use nvme-core workqueue

Instead of each transport using it's own workqueue, export
a single nvme-core workqueue and use that instead.

In the future, this will help us moving towards some unification
if controller setup/teardown flows.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
diff --git a/drivers/nvme/host/nvme.h b/drivers/nvme/host/nvme.h
index e2e341b..80e9adc 100644
--- a/drivers/nvme/host/nvme.h
+++ b/drivers/nvme/host/nvme.h
@@ -33,6 +33,8 @@ extern unsigned char shutdown_timeout;
 #define NVME_DEFAULT_KATO	5
 #define NVME_KATO_GRACE		10
 
+extern struct workqueue_struct *nvme_wq;
+
 enum {
 	NVME_NS_LBA		= 0,
 	NVME_NS_LIGHTNVM	= 1,