common/mlx5: free MR resource on device DMA unmap

common/mlx5: free MR resource on device DMA unmap

mlx5 PMD created the MR (Memory Region) resource on the
mlx5_dma_map call to make the memory available for DMA
operations. On the mlx5_dma_unmap call the MR resource
was not freed but inserted to MR Free list for further
garbage collection.
Actual MR resource destroying happened on device stop
call. That caused the runtime out of memory in case of
application performed multiple DMA map/unmap calls.

The fix immediately frees the MR resource on mlx5_dma_unmap
call not engaging the list. The export for mlx5_mr_free
function from common PMD part is added as well.

Fixes: 989e999d9305 ("net/mlx5: support PCI device DMA map and unmap")
Cc: stable@dpdk.org
Signed-off-by: Jiawei Wang <jiaweiw@nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>