osd/ReplicatedPG: verify we have enough data for WRITE and WRITEFULL

Fixes: #2207
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>
osd/ReplicatedPG: require write payload match length

Hopefully this won't break old clients; I can't think of any.  We *should*
be picky about our requests.

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