[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>

arch/alpha/kernel/osf_sys.c

@@ -96,6 +96,7 @@ struct osf_dirent {
 };
 
 struct osf_dirent_callback {
+	struct dir_context ctx;
 	struct osf_dirent __user *dirent;
 	long __user *basep;
 	unsigned int count;
@@ -155,8 +156,9 @@ SYSCALL_DEFINE4(osf_getdirentries, unsigned int, fd,
 	buf.basep = basep;
 	buf.count = count;
 	buf.error = 0;
+	buf.ctx.actor = osf_filldir;
 
-	error = vfs_readdir(arg.file, osf_filldir, &buf);
+	error = iterate_dir(arg.file, &buf.ctx);
 	if (error >= 0)
 		error = buf.error;
 	if (count != buf.count)