{net, IB}/mlx5: Replace mlx5_vzalloc with kvzalloc

Commit a7c3e901 ("mm: introduce kv[mz]alloc helpers") added
proper implementation of mlx5_vzalloc function to the MM core.

This made the mlx5_vzalloc function useless, so let's remove it.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>