convert the guts of nfs_direct_IO() to iov_iter

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/nfs_fs.h b/include/linux/nfs_fs.h
index 5a0d78e..0a82b6f 100644
--- a/include/linux/nfs_fs.h
+++ b/include/linux/nfs_fs.h
@@ -461,10 +461,10 @@
  */
 extern ssize_t nfs_direct_IO(int, struct kiocb *, struct iov_iter *, loff_t);
 extern ssize_t nfs_file_direct_read(struct kiocb *iocb,
-			const struct iovec *iov, unsigned long nr_segs,
+			struct iov_iter *iter,
 			loff_t pos, bool uio);
 extern ssize_t nfs_file_direct_write(struct kiocb *iocb,
-			const struct iovec *iov, unsigned long nr_segs,
+			struct iov_iter *iter,
 			loff_t pos, bool uio);
 
 /*