Update utils.py to forward size parameter in RawPcapnpReader (#4721)

I found this little flaw when reading packages from a pcap that was captured on a different device. This fixed the issue of raw data being truncated. If there is a good reason to ignore the size in here lmk

Author: StempunkDev

scapy/utils.py

@@ -1786,7 +1786,7 @@ def _read_packet(self, size=MTU):  # type: ignore
 
         """
         while True:
-            res = self._read_block()
+            res = self._read_block(size=size)
             if res is not None:
                 return res