staging: lustre: ldlm: restore some of the interval functionality

Earlier a bunch of interval handling got removed since it wasn't
used by the upstream client. Now some of it is needed again for
the client code so this patch restores what is needed.

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