Btrfs: reserve sufficient space for ioctl clone

Fix a crash/BUG_ON in the clone ioctl due to insufficient reservation. We
need to reserve space for:

 - adjusting the old extent (possibly splitting it)
 - adding the new extent
 - updating the inode

Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ioctl.c

@@ -2336,7 +2336,12 @@ static noinline long btrfs_ioctl_clone(struct file *file, unsigned long srcfd,
 			else
 				new_key.offset = destoff;
 
-			trans = btrfs_start_transaction(root, 1);
+			/*
+			 * 1 - adjusting old extent (we may have to split it)
+			 * 1 - add new extent
+			 * 1 - inode update
+			 */
+			trans = btrfs_start_transaction(root, 3);
 			if (IS_ERR(trans)) {
 				ret = PTR_ERR(trans);
 				goto out;