OpenZFS 9434 - Speculative prefetch is blocked by device removal code

OpenZFS 9434 - Speculative prefetch is blocked by device removal code

Device removal code does not set spa_indirect_vdevs_loaded for pools
that never experienced device removal.  At least one visual consequence
of it is completely blocked speculative prefetcher.  This patch sets
the variable in such situations.

Authored by: Alexander Motin <mav@FreeBSD.org>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tim Chase <tim@chase2k.com>
Approved by: Matt Ahrens <mahrens@delphix.com>
Ported-by: Giuseppe Di Natale <dinatale2@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/9434
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/16127b627b
Closes #7480