Simplify compat shims for /dev/crypto.

• Make session handling always use the CIOGSESSION2 structure. CIOGSESSION requests use a thunk similar to COMPAT_FREEBSD32 session requests. This permits the ioctl handler to use the 'crid' field unconditionally.

• Move COMPAT_FREEBSD32 handling out of the main ioctl handler body and instead do conversions in/out of thunk structures in dedicated blocks at the start and end of the ioctl function.

Reviewed by: markj (earlier version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D26178