arch/alpha: replace cpumask_weight with cpumask_empty where appropriate

common_shutdown_1() calls cpumask_weight() to check if any bit of a
given cpumask is set. We can do it more efficiently with cpumask_empty()
because cpumask_empty() stops traversing the cpumask as soon as it finds
first set bit, while cpumask_weight() counts all bits unconditionally.
Signed-off-by: NYury Norov <yury.norov@gmail.com>

arch/alpha/kernel/process.c

@@ -125,7 +125,7 @@ common_shutdown_1(void *generic_ptr)
 	/* Wait for the secondaries to halt. */
 	set_cpu_present(boot_cpuid, false);
 	set_cpu_possible(boot_cpuid, false);
-	while (cpumask_weight(cpu_present_mask))
+	while (!cpumask_empty(cpu_present_mask))
 		barrier();
 #endif