MFC r261618:

In g_eli_crypto_hmac_init(), zero out after using the ipad buffer,
k_ipad.

Note that the two consumers in geli(4) are not affected by this
issue because the way the code is constructed and as such, we
believe there is no security impact with or without this change
with geli(4)'s usage.

Reported by:	Serge van den Boom <serge vdboom.org>
Reviewed by:	pjd