Btrfs: remove negative dentry when deleting subvolumne

The use of btrfs_dentry_delete is removing dentries from the
dcache when deleting subvolumne. btrfs_dentry_delete ignores
negative dentries. This is incorrect since if we don't remove
the negative dentry, its parent dentry can't be removed.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 9a780c8..e8795be 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -830,6 +830,7 @@
 out_unlock:
 	mutex_unlock(&inode->i_mutex);
 	if (!err) {
+		shrink_dcache_sb(root->fs_info->sb);
 		btrfs_invalidate_inodes(dest);
 		d_delete(dentry);
 	}