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>