[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/scsi/scsi_lib.c

@@ -31,7 +31,7 @@
 
 
 #define SG_MEMPOOL_NR		ARRAY_SIZE(scsi_sg_pools)
-#define SG_MEMPOOL_SIZE		32
+#define SG_MEMPOOL_SIZE		2
 
 struct scsi_host_sg_pool {
 	size_t		size;