DM RAID: Fix comparison of index and quantity for "rebuild" parameter

The "rebuild" parameter takes an index argument that starts counting from
zero.  The conditional used to validate the index was using '>' rather than
'>=', leaving the door open for an index value that would be 1 too large.

Reported-by: Neil Brown <neilb@suse.de>
Signed-off-by: Jonathan Brassow <jbrassow@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>