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

module/zfs/vdev.c

@@ -488,6 +488,11 @@ vdev_alloc(spa_t *spa, vdev_t **vdp, nvlist_t *nv, vdev_t *parent, uint_t id,
 	if (nvlist_lookup_string(nv, ZPOOL_CONFIG_PHYS_PATH,
 	    &vd->vdev_physpath) == 0)
 		vd->vdev_physpath = spa_strdup(vd->vdev_physpath);
+
+	if (nvlist_lookup_string(nv, ZPOOL_CONFIG_VDEV_ENC_SYSFS_PATH,
+	    &vd->vdev_enc_sysfs_path) == 0)
+		vd->vdev_enc_sysfs_path = spa_strdup(vd->vdev_enc_sysfs_path);
+
 	if (nvlist_lookup_string(nv, ZPOOL_CONFIG_FRU, &vd->vdev_fru) == 0)
 		vd->vdev_fru = spa_strdup(vd->vdev_fru);
 
@@ -673,6 +678,10 @@ vdev_free(vdev_t *vd)
 		spa_strfree(vd->vdev_devid);
 	if (vd->vdev_physpath)
 		spa_strfree(vd->vdev_physpath);
+
+	if (vd->vdev_enc_sysfs_path)
+		spa_strfree(vd->vdev_enc_sysfs_path);
+
 	if (vd->vdev_fru)
 		spa_strfree(vd->vdev_fru);
 

module/zfs/vdev_label.c

@@ -375,6 +375,10 @@ vdev_config_generate(spa_t *spa, vdev_t *vd, boolean_t getstats,
 		fnvlist_add_string(nv, ZPOOL_CONFIG_PHYS_PATH,
 		    vd->vdev_physpath);
 
+	if (vd->vdev_enc_sysfs_path != NULL)
+		fnvlist_add_string(nv, ZPOOL_CONFIG_VDEV_ENC_SYSFS_PATH,
+		    vd->vdev_enc_sysfs_path);
+
 	if (vd->vdev_fru != NULL)
 		fnvlist_add_string(nv, ZPOOL_CONFIG_FRU, vd->vdev_fru);
 

module/zfs/zfs_fm.c

@@ -986,6 +986,12 @@ zfs_post_state_change(spa_t *spa, vdev_t *vd, uint64_t laststate)
 			    FM_EREPORT_PAYLOAD_ZFS_VDEV_PHYSPATH,
 			    vd->vdev_physpath);
 		}
+		if (vd->vdev_enc_sysfs_path) {
+			(void) nvlist_add_string(aux,
+			    FM_EREPORT_PAYLOAD_ZFS_VDEV_ENC_SYSFS_PATH,
+			    vd->vdev_enc_sysfs_path);
+		}
+
 		(void) nvlist_add_uint64(aux,
 		    FM_EREPORT_PAYLOAD_ZFS_VDEV_LASTSTATE, laststate);
 	}