messages: fix COMPAT_VERSION on MClientSession

This was incorrectly incremented to 2 by omission
of an explicit COMPAT_VERSION value.

Fixes: #9945

Signed-off-by: John Spray <john.spray@redhat.com>
(cherry picked from commit 1eb9bcb1d36014293efc687b4331be8c4d208d8e)
Merge pull request #2838 from ceph/wip-9945-giant

messages: fix COMPAT_VERSION on MClientSession

Reviewed-by: Sage Weil <sage@redhat.com>