GMAC: Reset initial hash value and counter in AES_GMAC_Reinit().

Previously, these values were only cleared in AES_GMAC_Init(), so a
second set of operations could reuse the final hash as the initial
hash. Currently this bug does not trigger in cryptosoft as existing
GMAC and GCM operations always use an on-stack auth context
initialized from a template context.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33315