drm/i915/userptr: reject zero user_size

drm/i915/userptr: reject zero user_size

commit c11c7bfd213495784b22ef82a69b6489f8d0092f upstream.

Operating on a zero sized GEM userptr object will lead to explosions.

Fixes: 5cc9ed4b9a7a ("drm/i915: Introduce mapping of user pages into video memory (userptr) ioctl")
Testcase: igt/gem_userptr_blits/input-checking
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20180502195021.30900-1-matthew.auld@intel.com
Cc: Loic <hackurx@opensec.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>