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>
Merge pull request #2837 from ceph/wip-9945

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