pnfs: record sequence in pnfs_layout_segment when it's created

In later patches, we're going to teach the client to be more selective
about how it returns layouts. This means keeping a record of what the
stateid's seqid was at the time that the server handed out a layout
segment.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/pnfs.h

@@ -64,6 +64,7 @@ struct pnfs_layout_segment {
 	struct list_head pls_lc_list;
 	struct pnfs_layout_range pls_range;
 	atomic_t pls_refcount;
+	u32 pls_seq;
 	unsigned long pls_flags;
 	struct pnfs_layout_hdr *pls_layout;
 	struct work_struct pls_work;