net/mlx5_core,mlx5_ib: Do not use vmap() on coherent memory

As David Daney pointed in mlx4_core driver [1], mlx5_core is also
misusing the DMA-API.

This patch is removing the code that vmap() memory allocated by
dma_alloc_coherent().

After this patch, users of this drivers might fail allocating resources
on memory fragmeneted systems.  This will be fixed later on.

[1] - https://patchwork.ozlabs.org/patch/458531/

CC: David Daney <david.daney@cavium.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>