nvmet: introduce the passthru Kconfig option

This patch updates KConfig file for the NVMeOF target where we add new
option so that user can selectively enable/disable passthru code.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
[logang@deltatee.com: fixed some of the wording in the help message]
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>

drivers/nvme/target/Kconfig

@@ -16,6 +16,18 @@ config NVME_TARGET
 	  To configure the NVMe target you probably want to use the nvmetcli
 	  tool from http://git.infradead.org/users/hch/nvmetcli.git.
 
+config NVME_TARGET_PASSTHRU
+	bool "NVMe Target Passthrough support"
+	depends on NVME_TARGET
+	depends on NVME_CORE=y || NVME_CORE=NVME_TARGET
+	help
+	  This enables target side NVMe passthru controller support for the
+	  NVMe Over Fabrics protocol. It allows for hosts to manage and
+	  directly access an actual NVMe controller residing on the target
+	  side, incuding executing Vendor Unique Commands.
+
+	  If unsure, say N.
+
 config NVME_TARGET_LOOP
 	tristate "NVMe loopback device support"
 	depends on NVME_TARGET