kern/175323: Fail to use ZVOL as a gmirror component

[Andriy Gapon]

This
> GEOM_MIRROR: Cannot add disk zvol/tank0/vol0 to mirror0 (error=45).

seems to be triggered by the following code in sys/geom/mirror/g_mirror.c:

error = g_getattr("GEOM::candelete", disk->d_consumer, &i);
if (error != 0)
        goto fail;

plus the fact that ZFS zvol does the following:
case BIO_GETATTR:
case BIO_DELETE:
default:
        g_io_deliver(bp, EOPNOTSUPP);
        break;

Perhaps, the gmirror code should be more flexible with respect to EOPNOTSUPP
from g_getattr?

-- 
Andriy Gapon