DAFS: Avoid logging harmless LEAVE_OFF failures

DAFS: Avoid logging harmless LEAVE_OFF failures

The DAFS volserver can create temporary destroyMe volumes, which it
then tells the fileserver to keep offline with an FSYNC_VOL_LEAVE_OFF
message. When the fileserver doesn't know that the volume exists,
currently it responds with SYNC_DENIED, which causes the volserver to
log the error with a "negative response on circuit FSSYNC" message.

These are harmless, though, since leaving a volume offline that the
fileserver does not know about is a no-op. So return the SYNC_FAILED
code instead, which does not get logged, and is more consistent with
some other FSYNC handlers like FSYNC_VOL_DONE.

Change-Id: I315d091fb419767ae33c1846d90e5ef83372477d
Reviewed-on: http://gerrit.openafs.org/3494
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>