Use a counter with a random base for explicit IVs in GCM.

This permits constructing the entire TLS header in ktls_frame() rather
than ktls_seq(). This also matches the approach used by OpenSSL which
uses an incrementing nonce as the explicit IV rather than the sequence
number.

Reviewed by: gallatin
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D22117