ceph: update ceph_msg_header structure

2 bytes of what was reserved space is now used by
userspace for the compat_version field.

Signed-off-by: John Spray <john.spray@redhat.com>
ceph: update CAPRELEASE message format

Version 2 includes the new osd epoch barrier
field.

Signed-off-by: John Spray <john.spray@redhat.com>