bcachefs: Clear fs_path_parent on subvolume unlink

This fixes recursive subvolume removal.

Subvolume deletion is asynchronous; fs_path_parent, and thus the entry
in the subvolume_children btree, need to be cleared when the subvolume
is unlinked from the fs heirarchy - else we'll spuriously think a
subvolume has children and deletion will fail.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/subvolume.c

@@ -561,6 +561,7 @@ int bch2_subvolume_unlink(struct btree_trans *trans, u32 subvolid)
 	}
 
 	SET_BCH_SUBVOLUME_UNLINKED(&n->v, true);
+	n->v.fs_path_parent = 0;
 	bch2_trans_iter_exit(trans, &iter);
 	return ret;
 }