staging: lustre: ldlm: restore interval_iterate_reverse function

Earlier the function interval_iterate_reverse function was
removed since it wasn't used by anyone. Now it is being
restored since it will be used by a future patch.

Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>