8969 Cannot boot from RAIDZ with parity > 1

illumos/illumos-gate@0fb055e81fd0cda5221da8ddd98b2f8d1fc6bdbe

At present it is possible to boot from a root pool that is on RAIDZ but not
one that is on RAIDZ2 or RAIDZ3. This is because, at the time the pool
version is checked to ensure support for dual/triple parity, the uberblock
has not yet been loaded into the SPA and therefore the code determines that
the pool version is too old and returns ENOTSUP.

Reviewed by: Igor Kozhukhov <igor@dilos.org>
Reviewed by: Andriy Gapon <avg@FreeBSD.org>
Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Andy Stormont <astormont@racktopsystems.com>
Reviewed by: Toomas Soome <tsoome@me.com>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Andy Fiddaman <omnios@citrus-it.co.uk>