8426 mark immutable buffer arguments as such in abd.h

illumos/illumos-gate@9b195260e22529ac0e2580faaf89402420589c1c
https://github.com/illumos/illumos-gate/commit/9b195260e22529ac0e2580faaf89402420589c1c

https://www.illumos.org/issues/8426

abd_copy_from_buf and abd_cmp_buf do not modify their void *buf arguments, so
qualify them with const.
abd_copy_from_buf_off and abd_cmp_buf_off already had that type for the
corresponding arguments.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>