selftests/seccomp: Avoid redundant register flushes

selftests/seccomp: Avoid redundant register flushes

When none of the registers have changed, don't flush them back. This can
happen if the architecture uses a non-register way to change the syscall
(e.g. arm64) , and a return value hasn't been written.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/lkml/20200912110820.597135-11-keescook@chromium.org
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>