xen/arm: missing put_cpu in xen_percpu_init

When CONFIG_PREEMPT is enabled, Linux will not be able to boot and warn:
[    4.127825] ------------[ cut here ]------------
[    4.133376] WARNING: at init/main.c:699 do_one_initcall+0x150/0x158()
[    4.140738] initcall xen_init_events+0x0/0x10c returned with preemption imbalance

This is because xen_percpu_init uses get_cpu but doesn't have the corresponding
put_cpu.
Signed-off-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

arch/arm/xen/enlighten.c

@@ -170,6 +170,7 @@ static void __init xen_percpu_init(void *unused)
 	per_cpu(xen_vcpu, cpu) = vcpup;
 
 	enable_percpu_irq(xen_events_irq, 0);
+	put_cpu();
 }
 
 static void xen_restart(char str, const char *cmd)