IB/mlx4: use kvmalloc_array to allocate wrid

We could use kvmalloc_array instead of the
kmalloc and __vmalloc combination.
After this we don't need to include linux/vmalloc.h

Signed-off-by: Li Dongyang <dongyang.li@anu.edu.au>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>