introduce iterate_dir() and dir_context

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>