audit: fix incorrect type of sessionid

The type of task->sessionid is unsigned int, the return
type of audit_get_sessionid should be consistent with it.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>