net/mlx5: Use 2-factor allocator calls

This restores the use of 2-factor allocation helpers that were already
fixed treewide. Please do not use open-coded multiplication; prefer,
instead, using 2-factor allocation helpers.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>