Fix volmode=none property behavior at import time

At import time spa_import() calls zvol_create_minors() directly: with
the current implementation we have no way to avoid device node
creation when volmode=none.

Fix this by enforcing volmode=none directly in zvol_alloc().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6426

module/zfs/zvol.c

@@ -1603,6 +1603,9 @@ zvol_alloc(dev_t dev, const char *name)
 	if (volmode == ZFS_VOLMODE_DEFAULT)
 		volmode = zvol_volmode;
 
+	if (volmode == ZFS_VOLMODE_NONE)
+		return (NULL);
+
 	zv = kmem_zalloc(sizeof (zvol_state_t), KM_SLEEP);
 
 	list_link_init(&zv->zv_next);