IB/mlx5: Use div64_u64 for num_var_hw_entries calculation

On i386:

ERROR: "__udivdi3" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!
ERROR: "__divdi3" [drivers/infiniband/hw/mlx5/mlx5_ib.ko] undefined!

Fixes: f164be8c ("IB/mlx5: Extend caps stage to handle VAR capabilities")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Reported-by: Alexander Lobakin <alobakin@dlink.ru>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/hw/mlx5/main.c

@@ -6545,7 +6545,7 @@ static int mlx5_ib_init_var_table(struct mlx5_ib_dev *dev)
 					doorbell_bar_offset);
 	bar_size = (1ULL << log_doorbell_bar_size) * 4096;
 	var_table->stride_size = 1ULL << log_doorbell_stride;
-	var_table->num_var_hw_entries = bar_size / var_table->stride_size;
+	var_table->num_var_hw_entries = div64_u64(bar_size, var_table->stride_size);
 	mutex_init(&var_table->bitmap_lock);
 	var_table->bitmap = bitmap_zalloc(var_table->num_var_hw_entries,
 					  GFP_KERNEL);