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

[Алексей Волков]

Eventually this patch works for me just fine.

--- sys/geom/mirror/g_mirror.c    (revision 245741)
+++ sys/geom/mirror/g_mirror.c    (working copy)
@@ -456,7 +456,7 @@
     disk->d_flags = md->md_dflags;
     error = g_getattr("GEOM::candelete", disk->d_consumer, &i);
     if (error != 0)
-        goto fail;
+        i=0;
     if (i)
         disk->d_flags |= G_MIRROR_DISK_FLAG_CANDELETE;
     if (md->md_provider[0] != '\0')