MFC r294813: MFV r294812:
6434 sa_find_sizes() may compute wrong SA header size

Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Andriy Gapon <avg@freebsd.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: James Pan <jiaming.pan@yahoo.com>

illumos/illumos-gate@3502ed6e7cb3f3d2e781960ab8fe465fdc884834