ns: Add c/r for /proc/$pid/ns/$ids references

Based on work done by Cyrill Corcunov (many thanks for that).

In this commit we implement c/r for files which have opened
/proc/$pid/ns/$ids entries.

The idea is rather simple one

Checkpoint
==========

- Check if the file name is the one of known to be ns ref
- If match then write protobuf entry

Restore
=======

- Read all ns entries from the image
- When criu tries to open one we lookup over process
  tree to figure out which PID should be used in path
  and then just open it

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/files.h

@@ -18,8 +18,19 @@ struct rst_info;
 struct parasite_ctl;
 
 struct fd_link {
-	char		name[PATH_MAX + 1];
-	size_t		len;
+	union {
+		/* Link info for generic file (path) */
+		struct {
+			char	name[PATH_MAX + 1];
+			size_t	len;
+		};
+
+		/* Link info for proc-ns file */
+		struct {
+			struct ns_desc *ns_d;
+			unsigned int ns_kid;
+		};
+	};
 };
 
 struct fd_parms {