loader: zfs toplevel vdev must have spa set.

[tsoome]

Summary

With salted checksums the salt setup is using spa to store runtime salt,
and for the IO, the spa is often fetched from vdev structure.

Unfortunately the vdev->spa is set only for leaf devices, which leaves
mirror and raidz pool toplevel devices with vdev->spa unset, and this
does break salt based checksums and will cause system reset while booting
from such pool.

Test Plan

Tested with 2+1 raidz and zpool create -O checksum=skein.

[lstewart]

With this patch applied on top of svn head r308477, I built a custom memstick image, wiped the harddrives and reran my build guide steps including creation of the RAIDZ zpool with "-O checksum=skein" option and can confirm the system boots, so although I did not test without this patch applied, I think it's safe to say the patch fixed things given that there have been no other relevant changes in sys/boot between the revision I tested with previously (r307747) and r308477.

[allanjude]

approved for commit

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