xen-gntdev: return -EFAULT on copy_to_user failure

copy_to_user() returns the amount of data remaining to be copied.  We
want to return a negative error code here.  The upper layers just
call WARN_ON() if we return non-zero so this doesn't change the
behavior.  But returning -EFAULT is still cleaner.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>