The compiler doesn't warn about this code:
       rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, SCMP_SYS(openat), 1,
           SCMP_CMP(0, SCMP_CMP_EQ, AT_FDCWD),
           SCMP_CMP(1, SCMP_CMP_EQ, param->value),
           SCMP_CMP(2, SCMP_CMP_EQ, O_RDONLY|...));
but note that the arg_cnt argument above is only 1.  This means that
only the first filter (argument 0 == AT_FDCWD) is actually checked!

This patch also fixes the above error in the openat() filter.
Earlier I fixed corresponding errors in filters for rename() and
mprotect().