Add new block group profile to store 3 copies in a simliar way that
current RAID1 does. The profile attributes and constraints are defined
in the raid table and used by the same code that already handles the
2-copy RAID1.

The minimum number of devices is 3, the maximum number of devices/chunks
that can be lost/damaged is 2. Like RAID6 but with 33% space
utilization.
Signed-off-by: David Sterba <dsterba@suse.com>