[BLOCK] Don't pin lots of memory in mempools

Currently we scale the mempool sizes depending on memory installed
in the machine, except for the bio pool itself which sits at a fixed
256 entry pre-allocation.

There's really no point in "optimizing" this OOM path, we just need
enough preallocated to make progress. A single unit is enough, lets
scale it down to 2 just to be on the safe side.

This patch saves ~150kb of pinned kernel memory on a 32-bit box.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

drivers/md/dm.c

@@ -1012,7 +1012,7 @@ static struct mapped_device *alloc_dev(int minor)
 	if (!md->tio_pool)
 		goto bad3;
 
-	md->bs = bioset_create(16, 16, 4);
+	md->bs = bioset_create(16, 16);
 	if (!md->bs)
 		goto bad_no_bioset;