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.

Cc: stable@vger.kernel.org
Fixes: 1c30e3af

 ("audit: add support for the openat2 syscall")
Reported-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>