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>