image: Introduce collect-nofree flag

Current collect helper frees the pb entry if there's
zero priv_size on cinfo. For files we'll have zero
priv_size (as entries will be collected by sub-cinfos),
while the entry in question should NOT be freed.
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/include/protobuf.h

@@ -36,6 +36,7 @@ struct collect_image_info {
 };
 
 #define COLLECT_SHARED		0x1	/* use shared memory for obj-s */
+#define COLLECT_NOFREE		0x2	/* don't free entry after callback */
 #define COLLECT_HAPPENED	0x4	/* image was opened and collected */
 
 extern int collect_image(struct collect_image_info *);

criu/protobuf.c

@@ -217,7 +217,7 @@ int collect_image(struct collect_image_info *cinfo)
 			break;
 		}
 
-		if (!cinfo->priv_size)
+		if (!cinfo->priv_size && !(cinfo->flags & COLLECT_NOFREE))
 			cr_pb_descs[cinfo->pb_type].free(msg, NULL);
 	}