[PATCH] blk: fix gendisk->in_flight accounting during barrier sequence

While executing barrrier sequence, the bar_rq which carries actual
write was accounted as normal IO on completion, while it wasn't on
queueing.  This caused gendisk->in_flight to be decremented by 1 after
each barrier thus messed up statistics.

This patch makes bar_rq not accounted as normal IO.  As the containing
barrier request as a whole is accounted, part of it shouldn't be.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>