Change the type of io_pktlen from uint16_t to unsigned int

I plan to use a simple IOB chain w/o IOB queue for tcp recv buffers.
for large configurations, uint16_t is not large enough to represent
the entire socket buffer.

include/nuttx/mm/iob.h

@@ -106,7 +106,7 @@ struct iob_s
   uint16_t io_len;      /* Length of the data in the entry */
   uint16_t io_offset;   /* Data begins at this offset */
 #endif
-  uint16_t io_pktlen;   /* Total length of the packet */
+  unsigned int io_pktlen; /* Total length of the packet */
 
   uint8_t  io_data[CONFIG_IOB_BUFSIZE];
 };