nfs41: serialize first layoutget of a file

Per RFC 5661 Errata 3208:
| A client MAY always forget its layout state and associated
| layout stateid at any time (See also section 12.5.5.1).
| In such case, the client MUST use a non-layout stateid for the next
| LAYOUTGET operation. This will signal the server that the client has
| no more layouts on the file and its respective layout state can be
| released before issuing a new layout in response to LAYOUTGET.

In order to make such a signal unique to server, client needs to serialize
all layoutgets using non-layout stateid. We implement this by serializing
layoutgets when client has no layout segments at hand.

Signed-off-by: Peng Tao <tao.peng@primarydata.com>
Signed-off-by: Tom Haynes <Thomas.Haynes@primarydata.com>

fs/nfs/pnfs.h

@@ -95,6 +95,7 @@ enum {
 	NFS_LAYOUT_ROC,			/* some lseg had roc bit set */
 	NFS_LAYOUT_RETURN,		/* Return this layout ASAP */
 	NFS_LAYOUT_INVALID_STID,	/* layout stateid id is invalid */
+	NFS_LAYOUT_FIRST_LAYOUTGET,	/* Serialize first layoutget */
 };
 
 enum layoutdriver_policy_flags {