samples/seccomp: Support programs with >256 instructions

Previously, the program size was incorrectly truncated to 8 bits,
resulting in broken labels in large programs. Also changes the jump
resolution loop to not rely on undefined behavior (making a pointer
point before the filter array).

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