mon/MonClient: skip CEPHX_V2 challenge if client doesn't support it

mon/MonClient: skip CEPHX_V2 challenge if client doesn't support it

If the client doesn't support the CEPHX_V2 challenge, and we don't require
it, skip it.  This allows the client to authenticate without getting an
error like

  cephx: verify_reply couldn't decrypt with error: error decoding block for decryption

Note that we don't have this problem in the monitor exchange in
Monitor::handle_auth_request() because that verify_authorizer() caller is
only used for msgrv2, and all such clients support CEPHX_V2.  Instead,
those client authenticate via the MAuth messages, a path that does not use
authorizers at all.

Fixes: https://tracker.ceph.com/issues/40716
Signed-off-by: Sage Weil <sage@redhat.com>