zed: allow limiting concurrent jobs

200ms time-out is relatively long, but if we already hit the cap,
then we'll likely be able to spawn multiple new jobs when we wake up

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11807

cmd/zed/zed_conf.h

@@ -39,6 +39,7 @@ struct zed_conf {
 	libzfs_handle_t	*zfs_hdl;		/* handle to libzfs */
 	int		zevent_fd;		/* fd for access to zevents */
 	char		*path;		/* custom $PATH for zedlets to use */
+	int16_t max_jobs;		/* max zedlets to run at one time */
 };
 
 struct zed_conf *zed_conf_create(void);