NFSv4: Add lease_time and lease_expired to 'nfs4:' line of mountstats

On the NFS client there is no low-impact way to determine the nfs4
lease time or whether the lease is expired, so add these to mountstats
with times displayed in seconds.

If the lease is not expired, display lease_expired=0. Otherwise,
display lease_expired=seconds_since_expired, similar to 'age:' line
in mountstats.
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/super.c

@@ -742,6 +742,16 @@ int nfs_show_options(struct seq_file *m, struct dentry *root)
 EXPORT_SYMBOL_GPL(nfs_show_options);
 
 #if IS_ENABLED(CONFIG_NFS_V4)
+static void show_lease(struct seq_file *m, struct nfs_server *server)
+{
+	struct nfs_client *clp = server->nfs_client;
+	unsigned long expire;
+
+	seq_printf(m, ",lease_time=%ld", clp->cl_lease_time / HZ);
+	expire = clp->cl_last_renewal + clp->cl_lease_time;
+	seq_printf(m, ",lease_expired=%ld",
+		   time_after(expire, jiffies) ?  0 : (jiffies - expire) / HZ);
+}
 #ifdef CONFIG_NFS_V4_1
 static void show_sessions(struct seq_file *m, struct nfs_server *server)
 {
@@ -850,6 +860,7 @@ int nfs_show_stats(struct seq_file *m, struct dentry *root)
 		seq_printf(m, ",acl=0x%x", nfss->acl_bitmask);
 		show_sessions(m, nfss);
 		show_pnfs(m, nfss);
+		show_lease(m, nfss);
 	}
 #endif