BACKPORT: binder: remove user_buffer_offset

Remove user_buffer_offset since there is no kernel
buffer pointer anymore.

(cherry pick from commit c41358a5f5217abd7c051e8d42397e5b80f3b3ed)
Bug: 67668716
Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Change-Id: I399219867704dc5013453a7738193c742fc970ad