net/mlx5: DR, Allocate icm_chunks from their own slab allocator

net/mlx5: DR, Allocate icm_chunks from their own slab allocator

SW steering allocates/frees lots of icm_chunk structs. To make this more
efficiently, create a separate kmem_cache and allocate these chunks from
this allocator.
By doing this we observe that the alloc/free "hiccups" frequency has
become much lower, which allows for a more steady rule insersion rate.

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