rbd: Correct readahead divide by zero exception

When readahead is used on old-format RBD images, a divide
by zero signal will be thrown.  This was caused by initializing
the readahead alignments prior to initializing the stripe layout
of old-format RBD images.

Fixes: 9857
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
Merge pull request #2770 from ceph/wip-9857

rbd: Correct readahead divide by zero exception

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>