staging/lustre/ldlm: In ldlm_pools_fini make sure there was init first.

It turns out if you call ldlm_pools_fini without completing the
ldlm_pools_init, then attempt to unregister not yet registered
shrinkers makes the kernel very unhappy.
So make sure we have them registered first.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>