messages: MForward: fix compat version

A while ago we bumped the head version and reset the compat version to 0.
Doing this so happens to make the messenger assume that the message does
not support the compat versioning and sets the compat version to the head
version -- thus making compat = 2 when it should have been 1.

The nasty side-effect of this is that upgrading from emperor to firefly
will have emperor-leaders being unable to decode forwarded messages from
firefly-peons.

Fixes: #8727

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #2082 from ceph/wip-8727

messages: MForward: fix compat version

Reviewed-by: Sage Weil <sage@redhat.com>