The device evacuation code adds a dependency that
vdev_compact_children() be able to properly empty the vdev_child
array by setting it to NULL and zeroing vdev_children. Under Linux,
kmem_alloc() and related functions return a sentinel pointer rather
than NULL for zero-sized allocations.
This is a part of ZoL port of device removal patch:
commit a1d477c24c7badc89c60955995fd84d311938486
Author: Matthew Ahrens <mahrens@delphix.com>
Ported-by: Tim Chase <tim@chase2k.com>
Approved by: re (kib)
MFC after: 1 week
178777f516