btrfs: sysfs: add devinfo/fsid to retrieve actual fsid from the device

In the case of the seed device, the fsid can be different from the mounted
sprout fsid.  The userland has to read the device superblock to know the
fsid but, that idea fails if the device is missing. So add a sysfs
interface devinfo/<devid>/fsid to show the fsid of the device.

For example:
  $ cd /sys/fs/btrfs/b10b02a5-f9de-4276-b9e8-2bfd09a578a8

  $ cat devinfo/1/fsid
  c44d771f-639d-4df3-99ec-5bc7ad2af93b
  $ cat  devinfo/3/fsid
  b10b02a5-f9de-4276-b9e8-2bfd09a578a8

Though it's related to seeding, the name of the sysfs file is plain fsid as it
matches what blkid says.  A path to the device's fsid will aid scripting.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -1537,6 +1537,16 @@ static ssize_t btrfs_devinfo_writeable_show(struct kobject *kobj,
 }
 BTRFS_ATTR(devid, writeable, btrfs_devinfo_writeable_show);
 
+static ssize_t btrfs_devinfo_fsid_show(struct kobject *kobj,
+				       struct kobj_attribute *a, char *buf)
+{
+	struct btrfs_device *device = container_of(kobj, struct btrfs_device,
+						   devid_kobj);
+
+	return sysfs_emit(buf, "%pU\n", device->fs_devices->fsid);
+}
+BTRFS_ATTR(devid, fsid, btrfs_devinfo_fsid_show);
+
 static ssize_t btrfs_devinfo_error_stats_show(struct kobject *kobj,
 		struct kobj_attribute *a, char *buf)
 {
@@ -1572,6 +1582,7 @@ BTRFS_ATTR(devid, error_stats, btrfs_devinfo_error_stats_show);
  */
 static struct attribute *devid_attrs[] = {
 	BTRFS_ATTR_PTR(devid, error_stats),
+	BTRFS_ATTR_PTR(devid, fsid),
 	BTRFS_ATTR_PTR(devid, in_fs_metadata),
 	BTRFS_ATTR_PTR(devid, missing),
 	BTRFS_ATTR_PTR(devid, replace_target),