SmallTable copies too few bytes when removing an item

[nding0405]

1. Problem
   The pointer version of remove() contains:

memmove(element,
        element + 1,
        size() - ((reinterpret_cast<uint8_t *>(element + 1) -
                   reinterpret_cast<uint8_t *>(base())) /
                  sizeof(T)));

[SmallTable::remove(T *)](https://github.com/CHERIoT-Platform/network-stack/blob/58425e6a1c65c4357b037aa817ba492e7a8fa70b/lib/firewall/firewall.cc#L275-L287)

The last value is the number of items left. but memmove() expects the number of bytes.

2. Why is it bad?
   If T is larger than one byte, the code copies too little data. This can damage the next firewall rules in the table.

This function is used here:

table.remove(&tuple);

[Firewall use](

network-stack/lib/firewall/firewall.cc

Lines 625 to 630 in 58425e6

	for (auto &tuple : table)
	{
	if (tuple.localPort == localPort)
	{
	table.remove(&tuple);
	break;

)

3. Suggested fix
   Multiply the number of items by `sizeof(T).