Don't try to get mg of hole vdev in removal

[pcd1193182]

Motivation and Context

When doing device removal, we verify that the vdev being removed isn't the last vdev in the normal class. To do that, we iterate over all the disks and checking their metaslab class until we find one in the normal group. Right now, if you have a hole vdev before the first allocatable normal class vdev, we kernel panic.

Description

Add a check for hole vdevs to the passivate code.

How Has This Been Tested?

Added a new test to the test suite and verified that it passes. Without the change, it fails.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[amotin]

I wonder if vdev_is_concrete(cvd) would be cleaner. May be in combination with && !vdev_is_dead(cvd).