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>