Turn on/off enclosure slot fault LED even when disk isn't present

Previously when a drive faulted, the statechange-led.sh script would lookup
the drive's LED sysfs entry in /sys/block/sd*/device/enclosure_device, and
turn it on.  During testing we noticed that if you pulled out a drive, or if
the drive was so badly broken that it no longer appeared to Linux, that the
/sys/block/sd* path would be removed, and the script could not lookup the
LED entry.

To fix this, this patch looks up the disks's more persistent
"/sys/class/enclosure/X:X:X:X/Slot N" LED sysfs path at pool import.  It then
passes that path to the statechange-led script to use, rather than having the
script look it up on the fly.  This allows the script to turn on/off the slot
LEDs even when the drive is missing.

Closes #5309 
Closes #2375

cmd/zed/zed_event.c

@@ -843,23 +843,6 @@ _zed_internal_event(const char *class, nvlist_t *nvl)
 	}
 }
 
-static void
-_zed_event_add_upath(uint64_t eid, zed_strings_t *zsp, nvlist_t *nvl)
-{
-	char *path = NULL;
-	char *upath = NULL;
-	if (nvlist_lookup_string(nvl, FM_EREPORT_PAYLOAD_ZFS_VDEV_PATH,
-	    &path) == 0) {
-		upath = get_underlying_path(NULL, path);
-		if (upath) {
-			_zed_event_add_var(eid, zsp, ZEVENT_VAR_PREFIX,
-			    "VDEV_UPATH",
-			    "%s", upath);
-			free(upath);
-		}
-	}
-}
-
 /*
  * Service the next zevent, blocking until one is available.
  */
@@ -932,16 +915,6 @@ zed_event_service(struct zed_conf *zcp)
 
 		_zed_event_add_time_strings(eid, zsp, etime);
 
-		/*
-		 * If a VDEV is included, resolve it's path to the "underlying
-		 * device".  This is useful for resolving device mapper and
-		 * multipath devices to their underlying /dev/sd* devices.
-		 * For example, if you have a DM or multipath VDEV
-		 * (/dev/mapper/mpatha) that points to one or more /dev/sd*
-		 * devices, this will return the first of its devices.
-		 */
-		_zed_event_add_upath(eid, zsp, nvl);
-
 		zed_exec_process(eid, class, subclass,
 		    zcp->zedlet_dir, zcp->zedlets, zsp, zcp->zevent_fd);