NFSv4: Set the SKIP_RENEW mount flag

This causes the NFSv4 client to only issue RENEW calls against the
server in case the client actually has files opened. In addition to
reducing the number of operations when idle, it has the advantage that
the server can purge its client state more aggressively.

pkg/filesystem/virtual/configuration/nfsv4_mount_darwin.go

@@ -149,10 +149,12 @@ func (m *nfsv4Mount) mount(terminationGroup program.Group, rpcServer *rpcserver.
 	attrMask[0] |= 1 << nfs_sys_prot.NFS_MATTR_FLAGS
 	flags := nfs_sys_prot.NfsMattrFlags{
 		Mask: []uint32{
-			1 << nfs_sys_prot.NFS_MFLAG_NOCALLBACK,
+			(1 << nfs_sys_prot.NFS_MFLAG_NOCALLBACK) |
+				(1 << nfs_sys_prot.NFS_MFLAG_SKIP_RENEW),
 		},
 		Value: []uint32{
-			1 << nfs_sys_prot.NFS_MFLAG_NOCALLBACK,
+			(1 << nfs_sys_prot.NFS_MFLAG_NOCALLBACK) |
+				(1 << nfs_sys_prot.NFS_MFLAG_SKIP_RENEW),
 		},
 	}
 	flags.WriteTo(&attrVals)