LoadPin: Return EFAULT on copy_from_user() failures

The copy_from_user() function returns the number of bytes remaining to
be copied on a failure. Such failures should return -EFAULT to high
levels.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 3f805f8c ("LoadPin: Enable loading from trusted dm-verity devices")
Cc: Matthias Kaehlcke <mka@chromium.org>
Cc: James Morris <jmorris@namei.org>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: linux-security-module@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>