drbd: account bitmap IO during resync as resync-(related-)-io

If we have a good resync rate, we will frequently update the on-disk
bitmap, which, if not accounted for as resync io, may let an otherwise
idle device appear to be "busy", and cause us to throttle resync.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_bitmap.c

@@ -991,6 +991,9 @@ static void bm_page_io_async(struct bm_aio_ctx *ctx, int page_nr, int rw) __must
 		bio_endio(bio, -EIO);
 	} else {
 		submit_bio(rw, bio);
+		/* this should not count as user activity and cause the
+		 * resync to throttle -- see drbd_rs_should_slow_down(). */
+		atomic_add(len >> 9, &mdev->rs_sect_ev);
 	}
 }