restore: set a dumpable flag after restoring changing uid and gid

This flag is reseted each time when uid-s, gid-s or capabilities
are changed.

------------------------ grep Error ------------------------
(00.235109)      6: Opening 0x002ad2fb7a1000-0x002ad2fb7a2000 0000000000000000 (101) vma
(00.235115)      6: Search for 0x002ad2fb7a1000 shmem 0x3ec72 0x2b9ac74058c0/5
(00.235119)      6: Waiting for the 3ec72 shmem to appear
(00.235122)      6: Opening shmem [/proc/5/fd/6]
(00.235153)      6: Error (criu/shmem.c:453): Can't open 5/fd/6 on procfs: Permission denied
(00.235171)      6: Error (criu/mem.c:1200): `- Can't open vma
(00.283466) uns: calling exit_usernsd (-1, 1)
(00.283521) uns: daemon calls 0x47f6b0 (189, -1, 1)
(00.283532) uns: `- daemon exits w/ 0
(00.289212) uns: daemon stopped
(00.289232) Error (criu/cr-restore.c:2152): Restoring FAILED.
------------------------ ERROR OVER ------------------------
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/files.c

@@ -674,6 +674,9 @@ int restore_fown(int fd, FownEntry *fown)
 		return -1;
 	}
 
+	if (prctl(PR_SET_DUMPABLE, 1, 0))
+		pr_perror("Unable to set PR_SET_DUMPABLE");
+
 	return 0;
 }