Generate IVs directly in esp_output.

This is the only place that uses CRYPTO_F_IV_GENERATE. All crypto
drivers currently duplicate the same boilerplate code to handle this
case. Doing the generation directly removes complexity from drivers.
It also simplifies support for separate input and output buffers.

Reviewed by: cem
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24449