percpu_ref: Replace kernel.h with the necessary inclusions

When kernel.h is used in the headers it adds a lot into dependency hell,
especially when there are circular dependencies are involved.

Replace kernel.h inclusion with the list of what is really being used.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

include/linux/percpu-refcount.h

@@ -51,9 +51,9 @@
 #define _LINUX_PERCPU_REFCOUNT_H
 
 #include <linux/atomic.h>
-#include <linux/kernel.h>
 #include <linux/percpu.h>
 #include <linux/rcupdate.h>
+#include <linux/types.h>
 #include <linux/gfp.h>
 
 struct percpu_ref;