nios2: fix __get_user()

commit 2e29f50a upstream.

a) should not leave crap on fault
b) should _not_ require access_ok() in any cases.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>