mds: bump protocol

In commit f689e5f049736bb0a0fa437e05936f6c1b9c1bb6 we change the encoding
and semantics for MDiscoverReply.

Signed-off-by: Sage Weil <sage@inktank.com>
mds: note MDiscoverReply encoding change in corpus

Signed-off-by: Sage Weil <sage@inktank.com>