percpu: add missing per_cpu_ptr_to_phys() definition for UP

Commit 3b034b0 implemented
per_cpu_ptr_to_phys() but forgot to add UP definition.  Add UP
definition which is simple wrapper around __pa().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>

include/linux/percpu.h

@@ -180,6 +180,11 @@ static inline void free_percpu(void *p)
 	kfree(p);
 }
 
+static inline phys_addr_t per_cpu_ptr_to_phys(void *addr)
+{
+	return __pa(addr);
+}
+
 static inline void __init setup_per_cpu_areas(void) { }
 
 static inline void *pcpu_lpage_remapped(void *kaddr)