mlx4: factorize page_address() calls

We need to compute the frame virtual address at different points.
Do it once.

Following patch will use the new va address for validate_loopback()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>