illumos gate

I've posted a patch for review here: https://reviews.csiden.org/r/110/

This problem can occur if an invalid/stale pool configuration is passed to the kernel as part of an import attempt. The code today assumes (incorrectly) that by the time vdev_load() is invoked that the configuration has been deemed valid but that's not true. It's possible that a configuration could be passed to the kernel that has extra vdevs in the configuration. Those vdevs may have come from an old pool that existed but has since been recreated. The example from the crash dump:

::spa -v

ADDR STATE NAME
ffffff051135d000 ACTIVE $import

ADDR             STATE     AUX          DESCRIPTION                        
    ffffff0511971900 CANT_OPEN CORRUPT_DATA root
    ffffff04ec444380 CANT_OPEN CORRUPT_DATA   missing
    ffffff04f7fcd680 CANT_OPEN CORRUPT_DATA   missing
    ffffff04f7fce940 CANT_OPEN CORRUPT_DATA   missing
    ffffff04f1a34640 CANT_OPEN CORRUPT_DATA   missing
    ffffff0510a28d40 CANT_OPEN CORRUPT_DATA   missing
    ffffff0510a280c0 HEALTHY   -              /dev/dsk/c2t5d0s0
    ffffff05ee585940 HEALTHY   -              missing
    ffffff05ee585300 HEALTHY   -              /dev/dsk/c2t8d0s0

Clearly, this pool configuration is a mess but the import was able to get far enough to call vdev_load() where it starts to load the metaslab associated with c2t5d0s0. This ends up trying to load the space_map for object 0x43 but that object doesn't exist resulting in a panic. A similar panic would occur if the object did exist but had a checksum error (again because it's stale).

In most case we would expect vdev_load() to just succeed as is seen in the example below:

1. zpool import
   pool: data
   id: 11689234226058225859
   state: ONLINE
   action: The pool can be imported using its name or numeric identifier.
   config:

   data ONLINE
   c1t1d0 ONLINE
   c1t3d0 ONLINE

   pool: data
   id: 7335367931613495462
   state: UNAVAIL
   status: One or more devices are missing from the system.
   action: The pool cannot be imported. Attach the missing
   devices and try again.
   see: http://illumos.org/msg/ZFS-8000-6X
   config:

   data         UNAVAIL  missing device
             c1t2d0     ONLINE

   Additional devices are known to be part of this pool, though their
           exact configuration cannot be determined.

Nonetheless, it's not guaranteed that we will be able to load the space_map or even the metaslab array so we must handle errors and bubble them up to vdev_load() to set the pool state accordingly.