net/local: Fix the error return length when read the bigger packet.

Verifiction:
  Write a bigger packet and read the packet using a smaller buffer.
  The return length of reading should be the length of smaller buffer.

Signed-off-by: liqinhui <liqinhui@xiaomi.com>

net/local/local_recvmsg.c

@@ -398,7 +398,6 @@ psock_dgram_recvfrom(FAR struct socket *psock, FAR void *buf, size_t len,
 
           DEBUGASSERT(tmplen <= remaining);
           remaining -= tmplen;
-          readlen += tmplen;
         }
       while (remaining > 0);
     }