IB/mlx5: use kvmalloc_array for mlx5_ib_wq

We observed multiple times on our Lustre OSS servers that when
the system memory is fragmented, kmalloc() in create_kernel_qp()
could fail order 4/5 allocations while we still have many free pages.

Switch to kvmalloc_array() to allow the operation to contine.

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