cxgb4: convert flower table to use rhashtable

T6 supports ~500K hash filters and can theoretically climb up to
~1 million hash filters. Preallocated hash table is not efficient
in terms of memory usage. So, use rhashtable instead which gives
the flexibility to grow based on usage.

Signed-off-by: Kumar Sanghvi <kumaras@chelsio.com>
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>