OpenZFS 9523 - Large alloc in zdb can cause trouble

OpenZFS 9523 - Large alloc in zdb can cause trouble

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

Authored by: Jorgen Lundman <lundman@lundman.net>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Porting Notes:
* Replaces an equivalent fix previously made for Linux.

OpenZFS-issue: https://illumos.org/issues/9523
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/2c1964a
Closes #7561