mount: add btrfs into fstypes

It's used by phys_stat_resolve_dev (broken by c5d2386a)
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -669,6 +669,9 @@ static struct fstype fstypes[] = {
 	}, {
 		.name = "simfs",
 		.code = FSTYPE__SIMFS,
+	}, {
+		.name = "btrfs",
+		.code = FSTYPE__UNSUPPORTED,
 	}
 };