[mesh-forwarder] allow last 6LoWPAN fragment to use longer length (#4945)

This commit relaxes the length check for the last 6LoWPAN fragment of
a message. The length of a 6LoWPAN fragment is truncated to be a
multiple of eight bytes (since fragment offset can be only expressed
as multiples of eight). However this requirement does not apply to the
last fragment. This change helps avoid an unnecessary extra fragment
for messages of certain length.

src/core/thread/mesh_forwarder.cpp

@@ -770,11 +770,11 @@ start:
         payload += fragmentHeaderLength;
         headerLength += fragmentHeaderLength;
 
-        fragmentLength = (aFrame.GetMaxPayloadLength() - headerLength) & ~0x7;
+        fragmentLength = aFrame.GetMaxPayloadLength() - headerLength;
 
         if (payloadLength > fragmentLength)
         {
-            payloadLength = fragmentLength;
+            payloadLength = (fragmentLength & ~0x7);
         }
 
         // Copy IPv6 Payload