dm raid: a few cleanups

- ensure maximum device limit in superblock
- rename DMPF_* (print flags) to CTR_FLAG_* (constructor flags)
  and their respective struct raid_set member
- use strcasecmp() in raid10_format_to_md_layout() as in the constructor

Signed-off-by: Heinz Mauelshagen <heinzm@redhat.com>
Reviewed-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>