net/mlx5: DR, Allocate htbl from its own slab allocator

SW steering allocates/frees lots of htbl structs. Create a
separate kmem_cache and allocate htbls from this allocator.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Alex Vesker <valex@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>