allow attaching fs_pin to a group not associated with some superblock

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/fs_pin.h b/include/linux/fs_pin.h
index a2e7191..2be38d1 100644
--- a/include/linux/fs_pin.h
+++ b/include/linux/fs_pin.h
@@ -7,4 +7,5 @@
 };
 
 void pin_remove(struct fs_pin *);
+void pin_insert_group(struct fs_pin *, struct vfsmount *, struct hlist_head *);
 void pin_insert(struct fs_pin *, struct vfsmount *);