macOS: race in registerDevice and offline

Which can cause "is busy" on the zvol.

Most easily tested with zfs-tester run over;

cli_root/zfs_create, zvol/zvol_cli, zvol/zvol_misc

which results in testpool/vol.33979-renamed "is busy".

This should be replaced with the proper "wait" condition, whatever
that may be.

module/os/macos/zfs/zvolIO.cpp

@@ -1102,6 +1102,7 @@ zvolRenameDevice(zvol_state_t *zv)
 	 * like zpool export.
 	 */
 	/* Inform clients of this device that name has changed */
+	delay(hz<<1);
 	if (zvol->offlineDevice() != 0 ||
 	    zvol->onlineDevice() != 0) {
 		dprintf("%s media reset failed\n", __func__);