DSA mod inverse fix

There is a side channel attack against the division used to calculate one of
the modulo inverses in the DSA algorithm.  This change takes advantage of the
primality of the modulo and Fermat's little theorem to calculate the inverse
without leaking information.

Thanks to Samuel Weiser for finding and reporting this.

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/7487)