ANDROID: dm: dm-user: New target that proxies BIOs to userspace

dm-user is essentially FUSE for block devices: as BIOs come in through
device mapper they are proxied to a userspace daemon via a control misc
device.

This is very much a work in progress.  There's a handful of FIXMEs
spread throughout the code with more details.  As far as I know there is
nothing broken with the current code, there's just more work to do.

Test: Ran the selftests on the version of this I'm developing for Linus'
      tree, on both 4.19 and 5.8.
Bug: 161496058
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: If5bcd8a43c3db5b556563ba303f474dd0d2902e8

drivers/md/Kconfig

@@ -601,4 +601,18 @@ config DM_BOW
 
 	  If unsure, say N.
 
+config DM_USER
+	tristate "Block device in userspace"
+	depends on BLK_DEV_DM
+	default y
+	help
+	  This device-mapper target allows a userspace daemon to provide the
+	  contents of a block device.  See
+	  <file:Documentation/block/dm-user.rst> for more information.
+
+	  To compile this code as a module, choose M here: the module will be
+	  called dm-user.
+
+	  If unsure, say N.
+
 endif # MD

drivers/md/Makefile

@@ -70,6 +70,7 @@ obj-$(CONFIG_DM_INTEGRITY)	+= dm-integrity.o
 obj-$(CONFIG_DM_ZONED)		+= dm-zoned.o
 obj-$(CONFIG_DM_WRITECACHE)	+= dm-writecache.o
 obj-$(CONFIG_DM_BOW)		+= dm-bow.o
+obj-$(CONFIG_DM_USER)		+= dm-user.o
 
 ifeq ($(CONFIG_DM_UEVENT),y)
 dm-mod-objs			+= dm-uevent.o