osd/PrimaryLogPG: assert write result == 0

osd/PrimaryLogPG: assert write result == 0

Historically, we have zeroed the result for any successful write op.
Soon, we will allow results >= 0 (e.g., from cls ops).  First, let's assert
these are zero, so we can catch any misbehaving users.

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