smp: Use str_plural() to fix Coccinelle warnings

Fixes the following two Coccinelle/coccicheck warnings reported by
string_choices.cocci:

	opportunity for str_plural(num_cpus)
	opportunity for str_plural(num_nodes)

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/r/20240508154225.309703-2-thorsten.blum@toblux.com