nvmet: add a generic NVMe target

This patch introduces a implementation of NVMe subsystems,
controllers and discovery service which allows to export
NVMe namespaces across fabrics such as Ethernet, FC etc.

The implementation conforms to the NVMe 1.2.1 specification
and interoperates with NVMe over fabrics host implementations.

Configuration works using configfs, and is best performed using
the nvmetcli tool from http://git.infradead.org/users/hch/nvmetcli.git,
which also has a detailed explanation of the required steps in the
README file.

Signed-off-by: Armen Baloyan <armenx.baloyan@intel.com>
Signed-off-by: Anthony Knapp <anthony.j.knapp@intel.com>
Signed-off-by: Jay Freyensee <james.p.freyensee@intel.com>
Signed-off-by: Ming Lin <ming.l@ssi.samsung.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

MAINTAINERS

@@ -8153,6 +8153,13 @@ S:	Supported
 F:	drivers/nvme/host/
 F:	include/linux/nvme.h
 
+NVM EXPRESS TARGET DRIVER
+M:	Christoph Hellwig <hch@lst.de>
+M:	Sagi Grimberg <sagi@grimberg.me>
+L:	linux-nvme@lists.infradead.org
+S:	Supported
+F:	drivers/nvme/target/
+
 NVMEM FRAMEWORK
 M:	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
 M:	Maxime Ripard <maxime.ripard@free-electrons.com>

drivers/nvme/Kconfig

@@ -1 +1,2 @@
 source "drivers/nvme/host/Kconfig"
+source "drivers/nvme/target/Kconfig"

drivers/nvme/Makefile

@@ -1,2 +1,3 @@
 
 obj-y		+= host/
+obj-y		+= target/

drivers/nvme/target/Kconfig

@@ -0,0 +1,16 @@
+
+config NVME_TARGET
+	tristate "NVMe Target support"
+	depends on BLOCK
+	depends on CONFIGFS_FS
+	help
+	  This enabled target side support for the NVMe protocol, that is
+	  it allows the Linux kernel to implement NVMe subsystems and
+	  controllers and export Linux block devices as NVMe namespaces.
+	  You need to select at least one of the transports below to make this
+	  functionality useful.
+
+	  To configure the NVMe target you probably want to use the nvmetcli
+	  tool from http://git.infradead.org/users/hch/nvmetcli.git.
+
+	  If unsure, say N.

drivers/nvme/target/Makefile

@@ -0,0 +1,5 @@
+
+obj-$(CONFIG_NVME_TARGET)		+= nvmet.o
+
+nvmet-y		+= core.o configfs.o admin-cmd.o io-cmd.o fabrics-cmd.o \
+			discovery.o