RDMA/mlx5: Prevent overflow in mmap offset calculations

The cmd and index variables declared as u16 and the result is supposed to
be stored in u64. The C arithmetic rules doesn't promote "(index >> 8) <<
16" to be u64 and leaves the end result to be u16.

Fixes: 7be76be ("IB/mlx5: Introduce VAR object and its alloc/destroy methods")
Link: https://lore.kernel.org/r/20200212072635.682689-10-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

Author: Leon Romanovsky

drivers/infiniband/hw/mlx5/main.c

@@ -2283,8 +2283,8 @@ static int mlx5_ib_mmap_offset(struct mlx5_ib_dev *dev,
 
 static u64 mlx5_entry_to_mmap_offset(struct mlx5_user_mmap_entry *entry)
 {
-	u16 cmd = entry->rdma_entry.start_pgoff >> 16;
-	u16 index = entry->rdma_entry.start_pgoff & 0xFFFF;
+	u64 cmd = (entry->rdma_entry.start_pgoff >> 16) & 0xFFFF;
+	u64 index = entry->rdma_entry.start_pgoff & 0xFFFF;
 
 	return (((index >> 8) << 16) | (cmd << MLX5_IB_MMAP_CMD_SHIFT) |
 		(index & 0xFF)) << PAGE_SHIFT;