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

[jhb]

Summary

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.

Test Plan

• tested with ktls_ocf with both cryptosoft and ccr
• used wireshark to verify incrementing nonces

[jhb]

Would you be able to test this with the current ISA-L module (and verify it works ok on your end)?

[gallatin]

Works fine running on a Netflix cache.