nfs41: .init_read and .init_write can be called with valid pg_lseg

With pgio refactoring in v3.15, .init_read and .init_write can be
called with valid pgio->pg_lseg. file layout was fixed at that time
by commit c6194271 (pnfs: filelayout: support non page aligned
layouts). But the generic helper still needs to be fixed.

Cc: stable@vger.kernel.org # 3.15+
Signed-off-by: Peng Tao <tao.peng@primarydata.com>

fs/nfs/pnfs.c

@@ -1711,8 +1711,7 @@ pnfs_generic_pg_init_read(struct nfs_pageio_descriptor *pgio, struct nfs_page *r
 {
 	u64 rd_size = req->wb_bytes;
 
-	WARN_ON_ONCE(pgio->pg_lseg != NULL);
-
+	if (pgio->pg_lseg == NULL) {
 		if (pgio->pg_dreq == NULL)
 			rd_size = i_size_read(pgio->pg_inode) - req_offset(req);
 		else
@@ -1724,6 +1723,7 @@ pnfs_generic_pg_init_read(struct nfs_pageio_descriptor *pgio, struct nfs_page *r
 						   rd_size,
 						   IOMODE_READ,
 						   GFP_KERNEL);
+	}
 	/* If no lseg, fall back to read through mds */
 	if (pgio->pg_lseg == NULL)
 		nfs_pageio_reset_read_mds(pgio);
@@ -1735,8 +1735,7 @@ void
 pnfs_generic_pg_init_write(struct nfs_pageio_descriptor *pgio,
 			   struct nfs_page *req, u64 wb_size)
 {
-	WARN_ON_ONCE(pgio->pg_lseg != NULL);
-
+	if (pgio->pg_lseg == NULL)
 		pgio->pg_lseg = pnfs_update_layout(pgio->pg_inode,
 						   req->wb_context,
 						   req_offset(req),