osd: fix 'ack' to be 'request_ack' in MOSDMarkMeDown

osd: fix 'ack' to be 'request_ack' in MOSDMarkMeDown

This field was passed along but always set to false.  It did not seem to
indicate whether this was/wasn't an ack (the reply message kept the same
value) or whether an ack was requested (it was always false but we always
reply).

Change this by bumping the message encoding, renaming the field, and
making old messages that are decoded force request_ack to true.  Reply
messages will always have request_ack false (although that part doesn't
matter much since the fact that it is a reply is clear from context).

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