staging: lustre: make struct lu_site_bkt_data private

This data structure only needs to be public so that
various modules can access a wait queue to wait for object
destruction.
If we provide a function to get the wait queue, rather than the
whole bucket, the structure can be made private.

Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>