btrfs: sysfs: show temp_fsid feature

This adds sysfs objects to indicate temp_fsid feature support and
its status.

  /sys/fs/btrfs/features/temp_fsid
  /sys/fs/btrfs/<UUID>/temp_fsid

For example:

   Consider two cloned and mounted devices.

      $ blkid /dev/sdc[1-2]
      /dev/sdc1: UUID="509ad44b-ad2a-4a8a-bc8d-fe69db7220d5" ..
      /dev/sdc2: UUID="509ad44b-ad2a-4a8a-bc8d-fe69db7220d5" ..

   One gets actual fsid, and the other gets the temp_fsid when
   mounted.

      $ btrfs filesystem show -m
      Label: none  uuid: 509ad44b-ad2a-4a8a-bc8d-fe69db7220d5
	      Total devices 1 FS bytes used 54.14MiB
	      devid    1 size 300.00MiB used 144.00MiB path /dev/sdc1

      Label: none  uuid: 33bad74e-c91b-43a5-aef8-b3cab97ae63a
	      Total devices 1 FS bytes used 54.14MiB
	      devid    1 size 300.00MiB used 144.00MiB path /dev/sdc2

   Their sysfs as below.

      $ cat /sys/fs/btrfs/features/temp_fsid
      0

      $ cat /sys/fs/btrfs/509ad44b-ad2a-4a8a-bc8d-fe69db7220d5/temp_fsid
      0

      $ cat /sys/fs/btrfs/33bad74e-c91b-43a5-aef8-b3cab97ae63a/temp_fsid
      1
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/sysfs.c

@@ -425,6 +425,13 @@ static ssize_t acl_show(struct kobject *kobj, struct kobj_attribute *a, char *bu
 }
 BTRFS_ATTR(static_feature, acl, acl_show);
 
+static ssize_t temp_fsid_supported_show(struct kobject *kobj,
+					struct kobj_attribute *a, char *buf)
+{
+	return sysfs_emit(buf, "0\n");
+}
+BTRFS_ATTR(static_feature, temp_fsid, temp_fsid_supported_show);
+
 /*
  * Features which only depend on kernel version.
  *
@@ -438,6 +445,7 @@ static struct attribute *btrfs_supported_static_feature_attrs[] = {
 	BTRFS_ATTR_PTR(static_feature, send_stream_version),
 	BTRFS_ATTR_PTR(static_feature, supported_rescue_options),
 	BTRFS_ATTR_PTR(static_feature, supported_sectorsizes),
+	BTRFS_ATTR_PTR(static_feature, temp_fsid),
 	NULL
 };
 
@@ -1205,6 +1213,15 @@ static ssize_t btrfs_generation_show(struct kobject *kobj,
 }
 BTRFS_ATTR(, generation, btrfs_generation_show);
 
+static ssize_t btrfs_temp_fsid_show(struct kobject *kobj,
+				    struct kobj_attribute *a, char *buf)
+{
+	struct btrfs_fs_info *fs_info = to_fs_info(kobj);
+
+	return sysfs_emit(buf, "%d\n", fs_info->fs_devices->temp_fsid);
+}
+BTRFS_ATTR(, temp_fsid, btrfs_temp_fsid_show);
+
 static const char * const btrfs_read_policy_name[] = { "pid" };
 
 static ssize_t btrfs_read_policy_show(struct kobject *kobj,
@@ -1307,6 +1324,7 @@ static const struct attribute *btrfs_attrs[] = {
 	BTRFS_ATTR_PTR(, read_policy),
 	BTRFS_ATTR_PTR(, bg_reclaim_threshold),
 	BTRFS_ATTR_PTR(, commit_stats),
+	BTRFS_ATTR_PTR(, temp_fsid),
 	NULL,
 };