pnfsblock: don't spinlock when freeing block_dev

bl_free_block_dev() may sleep. We can not call it with spinlock held.
Besides, there is no need to take bm_lock as we are last user freeing bm_devlist.

Cc: <stable@vger.kernel.org> raspberrypi#3.1+
Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/blocklayout/blocklayout.c

@@ -780,16 +780,13 @@ bl_cleanup_layoutcommit(struct nfs4_layoutcommit_data *lcdata)
 static void free_blk_mountid(struct block_mount_id *mid)
 {
 	if (mid) {
-		struct pnfs_block_dev *dev;
-		spin_lock(&mid->bm_lock);
-		while (!list_empty(&mid->bm_devlist)) {
-			dev = list_first_entry(&mid->bm_devlist,
-					       struct pnfs_block_dev,
-					       bm_node);
+		struct pnfs_block_dev *dev, *tmp;
+
+		/* No need to take bm_lock as we are last user freeing bm_devlist */
+		list_for_each_entry_safe(dev, tmp, &mid->bm_devlist, bm_node) {
 			list_del(&dev->bm_node);
 			bl_free_block_dev(dev);
 		}
-		spin_unlock(&mid->bm_lock);
 		kfree(mid);
 	}
 }