pnfs: serialize LAYOUTGET(openstateid)

We shouldn't send a LAYOUTGET(openstateid) unless all outstanding RPCs
using the previous stateid are completed.  This requires choosing the
stateid to encode earlier, so we can abort if one is not available (we
want to use the open stateid, but a LAYOUTGET is already out using
it), and adding a count of the number of outstanding rpc calls using
layout state (which for now consist solely of LAYOUTGETs).

Signed-off-by: Fred Isaman <iisaman@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/pnfs.h

@@ -69,6 +69,7 @@ struct pnfs_layout_hdr {
 	struct list_head	plh_layouts;   /* other client layouts */
 	struct list_head	plh_segs;      /* layout segments list */
 	nfs4_stateid		plh_stateid;
+	atomic_t		plh_outstanding; /* number of RPCs out */
 	unsigned long		plh_flags;
 	struct inode		*plh_inode;
 };