Retire the CRYPTO_F_IV_GENERATE flag.

The sole in-tree user of this flag has been retired, so remove this
complexity from all drivers. While here, add a helper routine drivers
can use to read the current request's IV into a local buffer. Use
this routine to replace duplicated code in nearly all drivers.

Reviewed by: cem
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D24450