configfs: call fsnotify_rmdir() hook

This will allow generating fsnotify delete events on unregister
of group/subsystem after the fsnotify_nameremove() hook is removed
from d_delete().

The rest of the d_delete() calls from this filesystem are either
called recursively from within debugfs_unregister_{group,subsystem},
called from a vfs function that already has delete hooks or are
called from shutdown/cleanup code.

Cc: Joel Becker <jlbec@evilplan.org>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>