audit: don't deref the syscall args when checking the openat2 open_how::flags

In the Linux kernel, the following vulnerability has been resolved: audit: don't deref the syscall args when checking the openat2 open_how::flags As reported by Jeff, dereferencing the openat2 syscall argument in audit_match_perm() to obtain the open_how::flags can result in an oops/page-fault. This patch fixes this by using the open_how struct that we store in the audit_context with audit_openat2_how(). Independent of this patch, Richard Guy Briggs posted a similar patch to the audit mailing list roughly 40 minutes after this patch was posted.