MFC r337179: 9523 Large alloc in zdb can cause trouble

MFC r337179: 9523 Large alloc in zdb can cause trouble

16MB alloc in zdb_embedded_block() can cause cores in certain situations
(clang, gcc55).

OsX commit: https://github.com/openzfsonosx/zfs/commit/ced236a5da6e72ea7bf6d2919fe14e17cffe10f1
FreeBSD commit: https://svnweb.freebsd.org/base?view=revision&revision=326150
illumos/illumos-gate@03a4c2f4bfaca30115963b76445279b36468a614

Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author:     Jorgen Lundman <lundman@lundman.net>

This is an update for r326150 (by avg), where this change comes from.