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>
(cherry picked from commit c92feebb81f90344a085f8256f0ed03cdc2f14e1)
Merge pull request #2083 from ceph/wip-8727.firefly

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