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

The blockdev layer has a concept of 'claiming' a device, so for example it
can be claimed when a filesystem is mounted or when it is included into a
raid array.  Only one subsystem can claim it at a time.

This patch matches this functionality available to user-space via the
O_EXCL flag to open.

This allows user-space programs to easily test if a device is currently
mounted etc, and to prevent a device from being mounted or otherwise
claimed.