Btrfs: Add drop inode func to avoid data=ordered deadlock

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 9ab4086..4deea39 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -425,6 +425,7 @@
 
 static struct super_operations btrfs_super_ops = {
 	.delete_inode	= btrfs_delete_inode,
+	.drop_inode	= btrfs_drop_inode,
 	.put_super	= btrfs_put_super,
 	.read_inode	= btrfs_read_locked_inode,
 	.write_super	= btrfs_write_super,