btrfs: drop unused parameter in mount_subvol

@device_name in mount_subvol() is not used, drop it.  Also see:
5bedc48a ("btrfs: drop unused parameters from mount_subvol").
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/super.c

@@ -1400,7 +1400,7 @@ static inline int is_subvolume_inode(struct inode *inode)
 }
 
 static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
-				   const char *device_name, struct vfsmount *mnt)
+				   struct vfsmount *mnt)
 {
 	struct dentry *root;
 	int ret;
@@ -1649,7 +1649,7 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
 	}
 
 	/* mount_subvol() will free subvol_name and mnt_root */
-	root = mount_subvol(subvol_name, subvol_objectid, device_name, mnt_root);
+	root = mount_subvol(subvol_name, subvol_objectid, mnt_root);
 
 out:
 	return root;