ARM: add get_user() support for 8 byte types

A new atomic modeset/pageflip ioctl being developed in DRM requires
get_user() to work for 64bit types (in addition to just put_user()).

v1: original
v2: pass correct size to check_uaccess, and better handling of narrowing
    double word read with __get_user_xb() (Russell King's suggestion)

Signed-off-by: Rob Clark <robdclark@gmail.com>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>