dm.c: Fix coercion bug in dm_merge_bvec()

dm_merge_bvec was originally added in f6fccb.  In this patch a value
in sectors is converted to bytes via a << 9, and then assigned to an
int.  This code made assumptions about the value of BIO_MAX_SECTORS.

A later patch, 148e51, removed the use of BIO_MAX_SECTORS.  At this
point the coercion resulted in a zero value.  The upshot being
dm_merge_bvec would only allow a single page to be added to a bio.

This patch is an interim patch.  The full solution needs to change
the max_size parameter in the dm_target merge methods to take a
sector_t (containing sectors, not bytes).

drivers/md/dm.c

@@ -1664,8 +1664,7 @@ static int dm_merge_bvec(struct request_queue *q,
 	struct mapped_device *md = q->queuedata;
 	struct dm_table *map = dm_get_live_table_fast(md);
 	struct dm_target *ti;
-	sector_t max_sectors;
-	int max_size = 0;
+	sector_t max_sectors, max_size = 0;
 
 	if (unlikely(!map))
 		goto out;
@@ -1680,16 +1679,22 @@ static int dm_merge_bvec(struct request_queue *q,
 	max_sectors = min(max_io_len(bvm->bi_sector, ti),
 			  (sector_t) queue_max_sectors(q));
 	max_size = (max_sectors << SECTOR_SHIFT) - bvm->bi_size;
-	if (unlikely(max_size < 0)) /* this shouldn't _ever_ happen */
-		max_size = 0;
+
+	/*
+	 * FIXME: This is horrible.  We should really be passing a sector_t
+	 * to the target merge functions (holding sectors not bytes).  Just
+	 * doing this as an interim fix.
+	 */
+	if (max_size > INT_MAX)
+		max_size = INT_MAX;
 
 	/*
 	 * merge_bvec_fn() returns number of bytes
 	 * it can accept at this offset
 	 * max is precomputed maximal io size
 	 */
 	if (max_size && ti->type->merge)
-		max_size = ti->type->merge(ti, bvm, biovec, max_size);
+		max_size = ti->type->merge(ti, bvm, biovec, (int) max_size);
 	/*
 	 * If the target doesn't support merge method and some of the devices
 	 * provided their merge_bvec method (we know this by looking for the