rcu: Fix CPU offload boot message when no CPUs are offloaded

In CONFIG_RCU_NOCB_CPU=y kernels, if the boot parameters indicate that
none of the CPUs should in fact be offloaded, the following somewhat
obtuse message appears:

	Offload RCU callbacks from CPUs: .

This commit therefore makes the message at least grammatically correct
in this case:

	Offload RCU callbacks from CPUs: (none)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>