dm raid: constructor fails on non-zero incompat_features

When lvm2 userspace requests a RaidLV repair, it sets the rebuild
constructor flag on the new replacement DataLVs but does not clear the
respective MetaLVs.  Hence the superblock that is loaded from such new
MetaLVs may have a non-zero incompat_features member and the constructor
will fail with false-positive on incompat_features.

Solve by initializing the incompat_features member properly.

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