dm raid: fix activation check in validate_raid_redundancy()

dm raid: fix activation check in validate_raid_redundancy()

During growing reshapes (i.e. stripes being added to a raid set), the
new stripe images are not in-sync and not part of the raid set until
the reshape is started.

LVM2 has to request multiple table reloads involving superblock updates
in order to reflect proper size of SubLVs in the cluster.  Before a stripe
adding reshape starts, validate_raid_redundancy() fails as a result of that
because it checks the total number of devices against the number of rebuild
ones rather than the actual ones in the raid set (as retrieved from the
superblock) thus resulting in failed raid4/5/6/10 redundancy checks.

E.g. convert 3 stripes -> 7 stripes raid5 (which only allows for maximum
1 device to fail) requesting +4 delta disks causing 4 devices to rebuild
during reshaping thus failing activation.

To fix this, move validate_raid_redundancy() to get access to the
current raid_set members.

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