xen: rename blkbk module xen-blkback.

blkbk is rather generic for a modular distro style kernel.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/blkback/Makefile

-obj-$(CONFIG_XEN_BLKDEV_BACKEND) := blkbk.o
+obj-$(CONFIG_XEN_BLKDEV_BACKEND) := xen-blkback.o
 
-blkbk-y	:= blkback.o xenbus.o interface.o vbd.o
+xen-blkback-y	:= blkback.o xenbus.o interface.o vbd.o