NFS: Add a helper to return a pointer to the open context of a struct nfs_page

Add a helper for when we remove the explicit pointer to the open
context.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/filelayout/filelayout.c

@@ -917,7 +917,7 @@ filelayout_pg_init_read(struct nfs_pageio_descriptor *pgio,
 	pnfs_generic_pg_check_layout(pgio);
 	if (!pgio->pg_lseg) {
 		pgio->pg_lseg = fl_pnfs_update_layout(pgio->pg_inode,
-						      req->wb_context,
+						      nfs_req_openctx(req),
 						      0,
 						      NFS4_MAX_UINT64,
 						      IOMODE_READ,
@@ -944,7 +944,7 @@ filelayout_pg_init_write(struct nfs_pageio_descriptor *pgio,
 	pnfs_generic_pg_check_layout(pgio);
 	if (!pgio->pg_lseg) {
 		pgio->pg_lseg = fl_pnfs_update_layout(pgio->pg_inode,
-						      req->wb_context,
+						      nfs_req_openctx(req),
 						      0,
 						      NFS4_MAX_UINT64,
 						      IOMODE_RW,