[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-crypt.c

@@ -867,7 +867,7 @@ static int crypt_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 		goto bad4;
 	}
 
-	cc->bs = bioset_create(MIN_IOS, MIN_IOS, 4);
+	cc->bs = bioset_create(MIN_IOS, MIN_IOS);
 	if (!cc->bs) {
 		ti->error = "Cannot allocate crypt bioset";
 		goto bad_bs;