Fix "multisegment RAID1, allocator uses one disk for both legs"

Fix "multisegment RAID1, allocator uses one disk for both legs"

In case of e.g. 3 PVs, creating or extending a RaidLV causes SubLV
collocation thus putting segments of diffent rimage (and potentially
larger rmeta) SubLVs onto the same PV.  For redundant RaidLVs this'll
compromise redundancy.  Fix by detecting such bogus allocation on
lvcreate/lvextend and reject the request.