md: remove unnecessary memset from multipath.

Recent commit bbba809e96539672f775a3d70102657d05816a5b
replaced mempool_create_kzalloc_pool with mempool_create_kmalloc_pool
plus a memset.
This memset is not needed (and we didn't need kzalloc in the first
place).
Ever field of the allocated structure (struct multipath_bh) is
initialised immediately except retry_list, and memset does not
initial a list_head anyway.

To remove the memset.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/multipath.c

@@ -150,7 +150,6 @@ static int multipath_make_request (struct request_queue *q, struct bio * bio)
 	}
 
 	mp_bh = mempool_alloc(conf->pool, GFP_NOIO);
-	memset(mp_bh, 0, sizeof(*mp_bh));
 
 	mp_bh->master_bio = bio;
 	mp_bh->mddev = mddev;