[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>

fs/dlm/lock.c

@@ -4429,7 +4429,8 @@ int dlm_user_unlock(struct dlm_ls *ls, struct dlm_user_args *ua_tmp,
 
 	if (lvb_in && ua->lksb.sb_lvbptr)
 		memcpy(ua->lksb.sb_lvbptr, lvb_in, DLM_USER_LVB_LEN);
-	ua->castparam = ua_tmp->castparam;
+	if (ua_tmp->castparam)
+		ua->castparam = ua_tmp->castparam;
 	ua->user_lksb = ua_tmp->user_lksb;
 
 	error = set_unlock_args(flags, ua, &args);
@@ -4474,7 +4475,8 @@ int dlm_user_cancel(struct dlm_ls *ls, struct dlm_user_args *ua_tmp,
 		goto out;
 
 	ua = (struct dlm_user_args *)lkb->lkb_astparam;
-	ua->castparam = ua_tmp->castparam;
+	if (ua_tmp->castparam)
+		ua->castparam = ua_tmp->castparam;
 	ua->user_lksb = ua_tmp->user_lksb;
 
 	error = set_unlock_args(flags, ua, &args);