[DLM] don't overwrite castparam if it's NULL

If the castaddr passed to the userland API is NULL then don't overwrite the
existing castparam. This allows a different thread to cancel a lock request and
the CANCEL AST gets delivered to the original thread.

bz#306391 (for RHEL4) refers.

Signed-Off-By: Patrick Caulfield <pcaulfie@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>