vfs: new superblock methods to override /proc/*/mount{s,info}

a) ->show_devname(m, mnt) - what to put into devname columns in mounts,
mountinfo and mountstats
b) ->show_path(m, mnt) - what to put into relative path column in mountinfo

Leaving those NULL gives old behaviour.  NFS switched to using those.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 13df14e..e6d3fe4 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1632,6 +1632,8 @@
 	void (*umount_begin) (struct super_block *);
 
 	int (*show_options)(struct seq_file *, struct vfsmount *);
+	int (*show_devname)(struct seq_file *, struct vfsmount *);
+	int (*show_path)(struct seq_file *, struct vfsmount *);
 	int (*show_stats)(struct seq_file *, struct vfsmount *);
 #ifdef CONFIG_QUOTA
 	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);