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/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 330d7b1..6462b31 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -1458,7 +1458,7 @@
 	ssize_t			ret;
 
 	if (rw & WRITE) {
-		size_t size = iov_length(iter->iov, iter->nr_segs);
+		size_t size = iov_iter_count(iter);
 
 		/*
 		 * We cannot preallocate a size update transaction here as we