kmem_zalloc(..., KM_SLEEP) will never fail

By definitition these allocations will never fail.  For
consistency with the rest of the code remove this dead error
handling code.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1558

module/zfs/zvol.c

@@ -1214,8 +1214,6 @@ zvol_alloc(dev_t dev, const char *name)
 	int error = 0;
 
 	zv = kmem_zalloc(sizeof (zvol_state_t), KM_SLEEP);
-	if (zv == NULL)
-		goto out;
 
 	zv->zv_queue = blk_init_queue(zvol_request, &zv->zv_lock);
 	if (zv->zv_queue == NULL)
@@ -1267,7 +1265,7 @@ out_queue:
 	blk_cleanup_queue(zv->zv_queue);
 out_kmem:
 	kmem_free(zv, sizeof (zvol_state_t));
-out:
+
 	return NULL;
 }