introduce iterate_dir() and dir_context

iterate_dir(): new helper, replacing vfs_readdir().

struct dir_context: contains the readdir callback (and will get more stuff
in it), embedded into whatever data that callback wants to deal with;
eventually, we'll be passing it to ->readdir() replacement instead of
(data,filldir) pair.

Change-Id: Idb3843e77b97d52490b297b1f9e7d7efab18eaef
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -1506,6 +1506,9 @@ int fiemap_check_flags(struct fiemap_extent_info *fieinfo, u32 fs_flags);
  * to have different dirent layouts depending on the binary type.
  */
 typedef int (*filldir_t)(void *, const char *, int, loff_t, u64, unsigned);
+struct dir_context {
+	filldir_t actor;
+};
 struct block_device_operations;
 
 /* These macros are for out of kernel modules to test that
@@ -2504,6 +2507,7 @@ loff_t inode_get_bytes(struct inode *inode);
 void inode_set_bytes(struct inode *inode, loff_t bytes);
 
 extern int vfs_readdir(struct file *, filldir_t, void *);
+extern int iterate_dir(struct file *, struct dir_context *);
 
 extern int vfs_stat(const char __user *, struct kstat *);
 extern int vfs_lstat(const char __user *, struct kstat *);