common/mlx5: fix memory leak in Windows MR deregistration

common/mlx5: fix memory leak in Windows MR deregistration

The "mlx5_devx_cmd_mkey_create" DevX cmd allocates DevX object using
mlx5_malloc and then creates MKey using glue function.
Compatibly, "mlx5_devx_cmd_destroy" cmd releases first the MKey using
glue function, and then free the DevX object using mlx5_free.

On Windows OS, the reg_mr function creates Mkey using
"mlx5_devx_cmd_mkey_create" cmd, but dereg_mr function using directly
glue function without freeing the object.
This behavior causes memory leak at each MR release.

In addition, the dereg_mr function makes sure that the MR pointer is
valid before destroying its fields, but always calls the memset function
that makes a difference to it.

This patch moves the dereg_mr function to use "mlx5_devx_cmd_destroy"
instead of glue function, and extends the validity test to the whole
function.

Fixes: ba420719823c ("common/mlx5: add reg/dereg MR on Windows")
Cc: stable@dpdk.org
Signed-off-by: Michael Baum <michaelba@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>