osd: recover degraded objects _before_ modifying it

osd: recover degraded objects _before_ modifying it

This will slow down writes to degraded objects because we will wait for it
to recover before applying the write.  OTOH it will be robust in the case
of large objects.  We can optimize the small object update (and overwrite)
cases later.

Signed-off-by: Sage Weil <sage@newdream.net>