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

include/sys/spa.h

@@ -1170,6 +1170,8 @@ extern void spa_configfile_set(spa_t *, nvlist_t *, boolean_t);
 /* asynchronous event notification */
 extern void spa_event_notify(spa_t *spa, vdev_t *vdev, nvlist_t *hist_nvl,
     const char *name);
+extern void zfs_ereport_zvol_post(const char *subclass, const char *name,
+    const char *device_name, const char *raw_name);
 
 /* waiting for pool activities to complete */
 extern int spa_wait(const char *pool, zpool_wait_activity_t activity,