crypto: create function to initialize encrypt and decrypt key

crypto: create function to initialize encrypt and decrypt key

Instead of always initialize the encrypt and decrypt keys separately,
implement an helper function init_key_ctx_bi() that takes care of
both of them for us.

Reduces code duplication and improves readability.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Antonio Quartulli <a@unstable.cc>
Message-Id: <20170707044704.7239-1-a@unstable.cc>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15011.html
Signed-off-by: David Sommerseth <davids@openvpn.net>
(cherry picked from commit 974513ea64020c956b531b1cabd76fdbac6655d8)