new helper: iterate_supers()

... and switch the simple "loop over superblocks and do something"
loops to it.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 5230867..eeb04ba 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2324,6 +2324,7 @@
 extern struct super_block *get_active_super(struct block_device *bdev);
 extern struct super_block *user_get_super(dev_t);
 extern void drop_super(struct super_block *sb);
+extern void iterate_supers(void (*)(struct super_block *, void *), void *);
 
 extern int dcache_dir_open(struct inode *, struct file *);
 extern int dcache_dir_close(struct inode *, struct file *);