mds/MDSMap: drop trailing else in favor of early return

This keeps the old-version special cases in one place and make it obvious
what the current/forward-looking path is.

Signed-off-by: Sage Weil <sage@redhat.com>
mds/MDSMap: fix incompat version for encoding

Back in 8f7900a09c8e490c9cd3a6f92ed1f0eb1f47f2a9 we added the new fields
before the 'extended' section, which made the encoding incompatible.
Instead, add them at the end--old clients don't care whether the enabled
flag is set or what the 'fs name' is.

Fixes: #8725
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2254 from ceph/wip-8725

mds: fix MDSMap encoding to be backward-compatible

Reviewed-by: Loic Dachary <loic@dachary.org>
Reviewed-by: John Spray <john.spray@redhat.com>