osd: EIO if a full-object read produces a bad digest

osd: EIO if a full-object read produces a bad digest

Add a tunable to control this.  Returning an EIO instead of garbled data
may not be the best approach in all environments.

Eventually we want to do something better here, like read from a replica
instead.

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