xen-blkback: use kzalloc() in favor of kmalloc()+memset()

This fixes the problem of three of those four memset()-s having
improper size arguments passed: Sizeof a pointer-typed expression
returns the size of the pointer, not that of the pointed to data.

It also reverts using kmalloc() instead of kzalloc() for the allocation
of the pending grant handles array, as that array gets fully
initialized in a subsequent loop.
Reported-by: NJulia Lawall <julia@diku.dk>
Signed-off-by: NJan Beulich <jbeulich@novell.com>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/block/xen-blkback/blkback.c

@@ -823,9 +823,9 @@ static int __init xen_blkif_init(void)
 
 	mmap_pages = xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST;
 
-	blkbk->pending_reqs          = kmalloc(sizeof(blkbk->pending_reqs[0]) *
+	blkbk->pending_reqs          = kzalloc(sizeof(blkbk->pending_reqs[0]) *
 					xen_blkif_reqs, GFP_KERNEL);
-	blkbk->pending_grant_handles = kzalloc(sizeof(blkbk->pending_grant_handles[0]) *
+	blkbk->pending_grant_handles = kmalloc(sizeof(blkbk->pending_grant_handles[0]) *
 					mmap_pages, GFP_KERNEL);
 	blkbk->pending_pages         = kzalloc(sizeof(blkbk->pending_pages[0]) *
 					mmap_pages, GFP_KERNEL);
@@ -848,8 +848,6 @@ static int __init xen_blkif_init(void)
 	if (rc)
 		goto failed_init;
 
-	memset(blkbk->pending_reqs, 0, sizeof(blkbk->pending_reqs));
-
 	INIT_LIST_HEAD(&blkbk->pending_free);
 	spin_lock_init(&blkbk->pending_free_lock);
 	init_waitqueue_head(&blkbk->pending_free_wq);