From: NeilBrown <neilb@cse.unsw.edu.au>

Current devices can be 'claimed' by filesystems (when mounting) or
md/raid (when being included in an array) or 'raw' or ....
This stop concurrent access by these systems.

However it is still possible for one system to claim the whole device
and a second system to claim one partition, which is not good.

With this patch, when a partition is claimed, the whole device is 
claimed for partitioning.  So you cannot have a partition and the
whole devices claimed at the same time (except if the whole device
is claimed for partitioning).