brd: Fix discard request processing

Avoid that discard requests with size => PAGE_SIZE fail with
-EIO. Refuse discard requests if the discard size is not a
multiple of the page size.

Fixes: 2dbe5495 ("brd: Refuse improperly aligned discard requests")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Jan Kara <jack@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Robert Elliot <elliott@hp.com>
Cc: stable <stable@vger.kernel.org> # v4.4+
Signed-off-by: Jens Axboe <axboe@fb.com>