Upstream: Add snapshot and zvol events

For kernel to send snapshot mount/unmount events to zed.

For kernel to send symlink creates/removes on zvol plumbing.
(/dev/run/dsk/zvol/$pool/$zvol -> /dev/diskX)

If zed misses the ENODEV, all errors after are EINVAL. Treat any error
as kernel module failure.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12416

cmd/zed/zed.c

@@ -291,7 +291,7 @@ idle:
 		rv = zed_event_service(&zcp);
 
 		/* ENODEV: When kernel module is unloaded (osx) */
-		if (rv == ENODEV)
+		if (rv != 0)
 			break;
 	}