MFC: r259428

Clear content of keyfiles loaded by the loader after processing them.

MFC: r259429

Clear some more places with potentially sensitive data.

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

MFC: r259428

Clear content of keyfiles loaded by the loader after processing them.

MFC: r259429

Clear some more places with potentially sensitive data.

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