NFS: report how long an NFS file system has been mounted

Add a field in nfs_server to record a timestamp when a mount succeeds.
Report the number of seconds the file system has been mounted via
nfs_show_stats().

Test plan:
Mount an NFS file system, watch the mountstats reports and compare with
clock time.

Signed-off-by: Chuck Lever <cel@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/inode.c b/fs/nfs/inode.c
index 8ee7411..1b2d782 100644
--- a/fs/nfs/inode.c
+++ b/fs/nfs/inode.c
@@ -299,6 +299,9 @@
 		goto out_no_root;
 	}
 
+	/* mount time stamp, in seconds */
+	server->mount_time = jiffies;
+
 	/* Get some general file system info */
 	if (server->namelen == 0 &&
 	    server->rpc_ops->pathconf(server, &server->fh, &pathinfo) >= 0)
@@ -674,6 +677,8 @@
 	seq_puts(m, mnt->mnt_sb->s_flags & MS_NODIRATIME ? ",nodiratime" : "");
 	nfs_show_mount_options(m, nfss, 1);
 
+	seq_printf(m, "\n\tage:\t%lu", (jiffies - nfss->mount_time) / HZ);
+
 	seq_printf(m, "\n\tcaps:\t");
 	seq_printf(m, "caps=0x%x", nfss->caps);
 	seq_printf(m, ",wtmult=%d", nfss->wtmult);