btrfs: update comment for fs_devices::seed_list in btrfs_rm_device

Update it since commit 944d3f9f ("btrfs: switch seed device to
list api") did conversion from fs_devices::seed to fs_devices::seed_list.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Su Yue <l@damenly.su>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -2159,7 +2159,7 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
 	/*
 	 * In normal cases the cur_devices == fs_devices. But in case
 	 * of deleting a seed device, the cur_devices should point to
-	 * its own fs_devices listed under the fs_devices->seed.
+	 * its own fs_devices listed under the fs_devices->seed_list.
 	 */
 	cur_devices = device->fs_devices;
 	mutex_lock(&fs_devices->device_list_mutex);