This makes it easier to refactor the GCM code to operate on crypto_buffer_cursors rather than plain contiguous buffers, with the aim of minimizing the amount of copying and zeroing done today. No functional change intended. Reviewed by: jhb MFC after: 1 week Sponsored by: Ampere Computing Submitted by: Klara, Inc. Differential Revision: https://reviews.freebsd.org/D28500