kernel.h: add u64_to_user_ptr()

This function had copies in 3 different files. Unify them in kernel.h.

Cc: Joe Perches <joe@perches.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Acked-by: Daniel Vetter <daniel.vetter@intel.com>	[drm/i915/]
Acked-by: Rob Clark <robdclark@gmail.com>		[drm/msm/]
Acked-by: Lucas Stach <l.stach@pengutronix.de>		[drm/etinav/]
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/kernel.h

@@ -53,6 +53,13 @@
 
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))
 
+#define u64_to_user_ptr(x) (		\
+{					\
+	typecheck(u64, x);		\
+	(void __user *)(uintptr_t)x;	\
+}					\
+)
+
 /*
  * This looks more complex than it should be. But we need to
  * get the type for the ~ right in round_down (it needs to be