net/mlx5: Add MLX5_FLEXIBLE_INLEN to safely calculate cmd inlen

Some commands use a flexible array after a common header. Add a macro to
safely calculate the total input length of the command, detecting
overflows and printing errors with specific values when such overflows
happen.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>