block: Don't count_vm_events for discard bio in submit_bio.

block: Don't count_vm_events for discard bio in submit_bio.

commit 1b99973f1c82707e46e8cb9416865a1e955e8f8c upstream.

In submit_bio, we count vm events by check READ/WRITE.
But actually DISCARD_NOBARRIER also has the WRITE flag set.
It looks as if in blkdev_issue_discard, we also add a
page as the payload and the bio_has_data check isn't enough.
So add another check for discard bio.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>