Have minimum version at 12.

Congestion threshold and max background do not require data structure
layout changes.

fuse/request_linux.go

@@ -4,6 +4,6 @@ const outputHeaderSize = 160
 
 const (
 	_FUSE_KERNEL_VERSION   = 7
-	_MINIMUM_MINOR_VERSION = 13
+	_MINIMUM_MINOR_VERSION = 12
 	_OUR_MINOR_VERSION     = 20
 )