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>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 30a6cc5..c46a025 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -7456,7 +7456,7 @@
 	 * we need to flush the dirty pages again to make absolutely sure
 	 * that any outstanding dirty pages are on disk.
 	 */
-	count = iov_length(iter->iov, iter->nr_segs);
+	count = iov_iter_count(iter);
 	if (test_bit(BTRFS_INODE_HAS_ASYNC_EXTENT,
 		     &BTRFS_I(inode)->runtime_flags))
 		filemap_fdatawrite_range(inode->i_mapping, offset, count);