block: rename blk-barrier.c to blk-flush.c

Without ordering requirements, barrier and ordering are minomers.
Rename block/blk-barrier.c to block/blk-flush.c.  Rename of symbols
will follow.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

block/Makefile

@@ -3,7 +3,7 @@
 #
 
 obj-$(CONFIG_BLOCK) := elevator.o blk-core.o blk-tag.o blk-sysfs.o \
-			blk-barrier.o blk-settings.o blk-ioc.o blk-map.o \
+			blk-flush.o blk-settings.o blk-ioc.o blk-map.o \
 			blk-exec.o blk-merge.o blk-softirq.o blk-timeout.o \
 			blk-iopoll.o blk-lib.o ioctl.o genhd.o scsi_ioctl.o