Split crp_buf into a union.

[jhb]

Summary

This adds explicit crp_mbuf and crp_uio pointers of the right type to
replace casts of crp_buf. This does not sweep through changing existing
code, but new code should use the correct fields instead of casts.

Test Plan

• tested with cryptocheck and the in-tree cryptotest tests against cryptosoft, aesni0, and ccr0

[kib]

As a note, anonymous union member is only in C11, for C99 this is a gnu extension.

[jhb]

Yes, though we already use anonymous unions in the tree now (e.g. struct mbuf uses them).