pnfs/blocklayout: calculate layoutupdate size correctly

We need to include the first u32 for the number of entries.  Add a helper
for the calculation instead of opencoding it so that it's in one place.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/blocklayout/extent_tree.c

@@ -462,6 +462,12 @@ ext_tree_mark_written(struct pnfs_block_layout *bl, sector_t start,
 	return err;
 }
 
+static size_t ext_tree_layoutupdate_size(size_t count)
+{
+	return sizeof(__be32) /* number of entries */ +
+		BL_EXTENT_SIZE * count;
+}
+
 static void ext_tree_free_commitdata(struct nfs4_layoutcommit_args *arg,
 		size_t buffer_size)
 {
@@ -489,7 +495,7 @@ static int ext_tree_encode_commit(struct pnfs_block_layout *bl, __be32 *p,
 			continue;
 
 		(*count)++;
-		if (*count * BL_EXTENT_SIZE > buffer_size) {
+		if (ext_tree_layoutupdate_size(*count) > buffer_size) {
 			/* keep counting.. */
 			ret = -ENOSPC;
 			continue;
@@ -530,7 +536,7 @@ ext_tree_prepare_commit(struct nfs4_layoutcommit_args *arg)
 	if (unlikely(ret)) {
 		ext_tree_free_commitdata(arg, buffer_size);
 
-		buffer_size = sizeof(__be32) + BL_EXTENT_SIZE * count;
+		buffer_size = ext_tree_layoutupdate_size(count);
 		count = 0;
 
 		arg->layoutupdate_pages =
@@ -549,7 +555,7 @@ ext_tree_prepare_commit(struct nfs4_layoutcommit_args *arg)
 	}
 
 	*start_p = cpu_to_be32(count);
-	arg->layoutupdate_len = sizeof(__be32) + BL_EXTENT_SIZE * count;
+	arg->layoutupdate_len = ext_tree_layoutupdate_size(count);
 
 	if (unlikely(arg->layoutupdate_pages != &arg->layoutupdate_page)) {
 		__be32 *p = start_p;