block: factor out __blkdev_issue_zero_pages()

commit 425a4dba7953e35ffd096771973add6d2f40d2ed upstream.

blkdev_issue_zeroout() will use this in !BLKDEV_ZERO_NOFALLBACK case.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Cc: Janne Huttunen <janne.huttunen@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>