MFV r322219: 8416 abd.h is not C++ friendly

FreeBSD note: this is a record-only commit, we had to apply the change
in r320156.

illumos/illumos-gate@5e2a074725cb7c16ea1c6554da11ab4d6b4e7aee
https://github.com/illumos/illumos-gate/commit/5e2a074725cb7c16ea1c6554da11ab4d6b4e7aee

https://www.illumos.org/issues/8416

A C++ compiler fails to compile abd_is_linear(), which is an inline function
defined in abd.h, with the following error:
     error: cannot initialize return object of type 'boolean_t' with an
     rvalue of type 'bool'
That happens because a bool can not be converted to an enum in C++.
That's a problem because abd.h can be visible through other header files that a
C++ program that works with ZFS can include.

Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Alek Pinchuk <pinchuk.alek@gmail.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Andriy Gapon <avg@FreeBSD.org>