bio: move the slab pointer inside the bio_set

In preparation for adding differently sized bios.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/linux/bio.h b/include/linux/bio.h
index d76e4bf..9340098 100644
--- a/include/linux/bio.h
+++ b/include/linux/bio.h
@@ -400,6 +400,7 @@
 #define BIOVEC_MAX_IDX	(BIOVEC_NR_POOLS - 1)
 
 struct bio_set {
+	struct kmem_cache *bio_slab;
 	mempool_t *bio_pool;
 #if defined(CONFIG_BLK_DEV_INTEGRITY)
 	mempool_t *bio_integrity_pool;