dlm: drop own rsb pre allocation mechanism

This patch drops the own written rsb pre allocation mechanism as this is
already done by using kmem caches, we don't need another layer on top of
that to running some pre allocation scheme.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>