coroipcs: Ensure rb data are not overwritten

With introduce of rb alignment (55600762), it's not enough to subtract
one from number of free bytes, but also alignment must be taken to
account. Easiest solution is to subtract 9 bytes.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Steven Dake <sdake@redhat.com>