dm raid: fix oops on upgrading to extended superblock format

When a RAID set was created on dm-raid version < 1.9.0 (old RAID
superblock format), all of the new 1.9.0 members of the superblock are
uninitialized (zero) -- including the device sectors member needed to
support shrinking.

All the other accesses to superblock fields new in 1.9.0 were reviewed
and verified to be properly guarded against invalid use.  The 'sectors'
member was the only one used when the superblock version is < 1.9.

Don't access the superblock's >= 1.9.0 'sectors' member unconditionally.
Also add respective comments.

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