RAID: Make 'vgreduce --removemissing' work with RAID LVs

RAID: Make 'vgreduce --removemissing' work with RAID LVs

Currently it is impossible to remove a failed PV which has a RAID LV
on it.  This patch fixes the issue by replacing the failed PV with an
'error' segment within the affected sub-LVs.  Once there is no longer
a RAID LV using the PV, it can be removed.

Most often, it is better to replace a failed RAID device with a spare.
(You can use 'lvconvert --repair <vg>/<LV>' to accomplish that.)
However, if there are no spares in the volume group and none will be
added, it is useful to be able to removed the failed device.

Following patches address the ability to perform 'lvconvert' operations
on RAID LVs that contain sub-LVs composed of 'error' segments.