RDMA/core: Add a helper API rdma_free_hw_stats_struct

Add a new API rdma_free_hw_stats_struct to pair with
rdma_alloc_hw_stats_struct (which is also de-inlined).

This will be useful when there are more alloc/free works in following
patches.

Link: https://lore.kernel.org/r/20211008122439.166063-5-markzhang@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>