image-desc: Use unsigned format for tmpfs image

The index comes from mnt_id which is signed integer
both in kernel and in userspace, but negative value
is never valid, thus don't use it.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/image-desc.c

@@ -76,7 +76,7 @@ struct cr_fd_desc_tmpl imgset_template[CR_FD_MAX] = {
 	FD_ENTRY_F(RULE,	"rule-%u", O_NOBUF),
 	FD_ENTRY_F(IPTABLES,	"iptables-%u", O_NOBUF),
 	FD_ENTRY_F(IP6TABLES,	"ip6tables-%u", O_NOBUF),
-	FD_ENTRY_F(TMPFS_IMG,	"tmpfs-%d.tar.gz", O_NOBUF),
+	FD_ENTRY_F(TMPFS_IMG,	"tmpfs-%u.tar.gz", O_NOBUF),
 	FD_ENTRY_F(TMPFS_DEV,	"tmpfs-dev-%u.tar.gz", O_NOBUF),
 	FD_ENTRY_F(AUTOFS,	"autofs-%u", O_NOBUF),
 	FD_ENTRY(BINFMT_MISC_OLD, "binfmt-misc-%u"),