cifs: no need to use rcu_assign_pointer on immutable keys

Neither keytype in use by CIFS has an "update" method. This means that
the keys are immutable once instantiated. We don't need to use RCU
to set the payload data pointers.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>