Make inline ABD predicates compatible with C++

FreeBSD's zfsd fails to build after e2af2acce3 due to strict type
checking errors from the implicit conversion between bool and boolean_t
in the inline predicate definitions in abd.h.

Use conditionals to return the correct value type from these functions.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #11592