drm/xe: Fix access_ok check in user_fence_create

drm/xe: Fix access_ok check in user_fence_create

Check size of the data not size of the pointer.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202407300421.IBkAja96-lkp@intel.com/
Fixes: 0fde907da2d5 ("drm/xe: Validate user fence during creation")
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Reviewed-by: Apoorva Singh <apoorva.singh@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240806110722.28661-1-nirmoy.das@intel.com
Signed-off-by: Nirmoy Das <nirmoy.das@intel.com>