[readdir] 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.

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

Documentation/filesystems/porting

@@ -445,3 +445,6 @@ object doesn't exist.  It's remote/distributed ones that might care...
 [mandatory]
 	FS_REVAL_DOT is gone; if you used to have it, add ->d_weak_revalidate()
 in your dentry operations instead.
+--
+[mandatory]
+	vfs_readdir() is gone; switch to iterate_dir() instead