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: openzfs/openzfs@16127b627b
Closes #7480

module/zfs/vdev_removal.c

@@ -352,6 +352,7 @@ spa_remove_init(spa_t *spa)
 		spa->spa_removing_phys.sr_state = DSS_NONE;
 		spa->spa_removing_phys.sr_removing_vdev = -1;
 		spa->spa_removing_phys.sr_prev_indirect_vdev = -1;
+		spa->spa_indirect_vdevs_loaded = B_TRUE;
 		return (0);
 	} else if (error != 0) {
 		return (error);