link: Fix MDU calculation size for Links

We need to substract the HEADER_MAXSIZE instead of MINSIZE to correctly
calculate the MDU of a Link. In order to avoid similar mistakes in the
future and not duplicate this calculation, we can reuse Reticulum.MDU
instead.

RNS/Link.py

@@ -61,7 +61,7 @@ class Link:
     ECPUBSIZE         = 32+32
     KEYSIZE           = 32
 
-    MDU = math.floor((RNS.Reticulum.MTU-RNS.Reticulum.IFAC_MIN_SIZE-RNS.Reticulum.HEADER_MINSIZE-RNS.Identity.FERNET_OVERHEAD)/RNS.Identity.AES128_BLOCKSIZE)*RNS.Identity.AES128_BLOCKSIZE - 1
+    MDU = math.floor((RNS.Reticulum.MDU-RNS.Identity.FERNET_OVERHEAD)/RNS.Identity.AES128_BLOCKSIZE)*RNS.Identity.AES128_BLOCKSIZE - 1
 
     ESTABLISHMENT_TIMEOUT_PER_HOP = RNS.Reticulum.DEFAULT_PER_HOP_TIMEOUT
     """