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>