libceph: 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>
Reviewed-by: Sage Weil <sage@redhat.com>
ceph: message versioning fixes

There were two places we were assigning version in host byte order
instead of network byte order.

Also in MSG_CLIENT_SESSION we weren't setting compat_version in the
header to reflect continued compatability with older MDSs.

Fixes: http://tracker.ceph.com/issues/9945

Signed-off-by: John Spray <john.spray@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>