Btrfs: add snapshot/subvolume destroy ioctl

This patch adds snapshot/subvolume destroy ioctl.  A subvolume that isn't being
used and doesn't contains links to other subvolumes can be destroyed.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6d6d06c..0242c8b 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -676,6 +676,7 @@
 }
 
 static struct super_operations btrfs_super_ops = {
+	.drop_inode	= btrfs_drop_inode,
 	.delete_inode	= btrfs_delete_inode,
 	.put_super	= btrfs_put_super,
 	.sync_fs	= btrfs_sync_fs,