NFSv4.1: Fix some issues with pnfs_generic_pg_test

1. If the intention is to coalesce requests 'prev' and 'req' then we
   have to ensure at least that we have a layout starting at
   req_offset(prev).

2. If we're only requesting a minimal layout of length desc->pg_count,
   we need to test the length actually returned by the server before
   we allow the coalescing to occur.

3. We need to deal correctly with (pgio->lseg == NULL)

4. Fixup the test guarding the pnfs_update_layout.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/objlayout/objio_osd.c

@@ -1000,6 +1000,9 @@ static bool objio_pg_test(struct nfs_pageio_descriptor *pgio,
 	if (!pnfs_generic_pg_test(pgio, prev, req))
 		return false;
 
+	if (pgio->pg_lseg == NULL)
+		return true;
+
 	return pgio->pg_count + req->wb_bytes <=
 			OBJIO_LSEG(pgio->pg_lseg)->max_io_size;
 }

fs/nfs/pnfs.c

@@ -1064,19 +1064,21 @@ pnfs_generic_pg_test(struct nfs_pageio_descriptor *pgio, struct nfs_page *prev,
 		gfp_flags = GFP_NOFS;
 	}
 
-	if (pgio->pg_count == prev->wb_bytes) {
+	if (pgio->pg_lseg == NULL) {
+		if (pgio->pg_count != prev->wb_bytes)
+			return true;
 		/* This is first coelesce call for a series of nfs_pages */
 		pgio->pg_lseg = pnfs_update_layout(pgio->pg_inode,
 						   prev->wb_context,
-						   req_offset(req),
+						   req_offset(prev),
 						   pgio->pg_count,
 						   access_type,
 						   gfp_flags);
-		return true;
+		if (pgio->pg_lseg == NULL)
+			return true;
 	}
 
-	if (pgio->pg_lseg &&
-	    req_offset(req) > end_offset(pgio->pg_lseg->pls_range.offset,
+	if (req_offset(req) > end_offset(pgio->pg_lseg->pls_range.offset,
 					 pgio->pg_lseg->pls_range.length))
 		return false;