selftests/seccomp: Allow syscall nr and ret value to be set separately

In preparation for setting syscall nr and ret values separately, refactor
the helpers to take a pointer to a value, so that a NULL can indicate
"do not change this respective value". This is done to keep the regset
read/write happening once and in one code path.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/lkml/20200921075031.j4gruygeugkp2zwd@wittgenstein/


Signed-off-by: Kees Cook <keescook@chromium.org>