MFV r337208: 9591 ms_shift can be incorrectly changed in MOS config for

MFV r337208: 9591 ms_shift can be incorrectly changed in MOS config for
indirect vdevs that have been historically expanded

illumos/illumos-gate@11f6a9680e013a7c9c57dc0b64d3e91e2eee1a6b

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <gwilson@zfsmail.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed by: Tim Chase <tim@chase2k.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author:     Serapheim Dimitropoulos <serapheim@delphix.com>