Add zp->z_is_zvol flag

A new flag is required for the zfs_rlock code to determine if
it is operation of the zvol of zpl dataset.  This used to be
keyed off the zp->z_vnode, which was a hack to begin with, but
with the removal of vnodes we needed a dedicated flag.

module/zfs/zfs_rlock.c

@@ -112,14 +112,14 @@ zfs_range_lock_writer(znode_t *zp, rl_t *new)
 		 * Range locking is also used by zvol and uses a
 		 * dummied up znode. However, for zvol, we don't need to
 		 * append or grow blocksize, and besides we don't have
-		 * a "sa" data or z_zfsvfs - so skip that processing.
+		 * a "sa" data or zfs_sb_t - so skip that processing.
 		 *
 		 * Yes, this is ugly, and would be solved by not handling
 		 * grow or append in range lock code. If that was done then
 		 * we could make the range locking code generically available
 		 * to other non-zfs consumers.
 		 */
-		if (zp->z_vnode) { /* caller is ZPL */
+		if (!zp->z_is_zvol) { /* caller is ZPL */
 			/*
 			 * If in append mode pick up the current end of file.
 			 * This is done under z_range_lock to avoid races.

module/zfs/zvol.c

@@ -1062,6 +1062,8 @@ zvol_alloc(dev_t dev, const char *name)
 	mutex_init(&zv->zv_znode.z_range_lock, NULL, MUTEX_DEFAULT, NULL);
 	avl_create(&zv->zv_znode.z_range_avl, zfs_range_compare,
 	    sizeof (rl_t), offsetof(rl_t, r_node));
+	zv->zv_znode.z_is_zvol = TRUE;
+
 	spin_lock_init(&zv->zv_lock);
 	list_link_init(&zv->zv_next);