get rid of pointless iov_length() in ->direct_IO()

all callers have iov_length(iter->iov, iter->nr_segs) == iov_iter_count(iter)

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

mm/filemap.c

@@ -1698,7 +1698,7 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
 			goto out; /* skip atime */
 		size = i_size_read(inode);
 		retval = filemap_write_and_wait_range(mapping, pos,
-					pos + iov_length(iov, nr_segs) - 1);
+					pos + count - 1);
 		if (!retval)
 			retval = mapping->a_ops->direct_IO(READ, iocb, &i, pos);