pnfs: allow splicing pre-encoded pages into the layoutcommit args

Currently there is no XDR buffer space allocated for the per-layout driver
layoutcommit payload, which leads to server buffer overflows in the
blocklayout driver even under simple workloads.  As we can't do per-layout
sizes for XDR operations we'll have to splice a previously encoded list
of pages into the XDR stream, similar to how we handle ACL buffers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
diff --git a/include/linux/nfs_xdr.h b/include/linux/nfs_xdr.h
index e563b2c..f4092c6 100644
--- a/include/linux/nfs_xdr.h
+++ b/include/linux/nfs_xdr.h
@@ -279,6 +279,9 @@
 	__u64 lastbytewritten;
 	struct inode *inode;
 	const u32 *bitmask;
+	size_t layoutupdate_len;
+	struct page *layoutupdate_page;
+	struct page **layoutupdate_pages;
 };
 
 struct nfs4_layoutcommit_res {