Btrfs: don't pass a trans handle unnecessarily in volumes.c

Some functions never use the transaction handle passed to them.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 8603ee4..5b53479 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -7084,7 +7084,7 @@
 		 * space to fit our block group in.
 		 */
 		if (device->total_bytes > device->bytes_used + min_free) {
-			ret = find_free_dev_extent(NULL, device, min_free,
+			ret = find_free_dev_extent(device, min_free,
 						   &dev_offset, NULL);
 			if (!ret)
 				dev_nr++;