IB/mlx4: Use vmalloc for WR buffers when needed

There are several hits that WR buffer allocation(kmalloc) failed.
It failed at order 3 and/or 4 contigous pages allocation. At the same time
there are actually 100MB+ free memory but well fragmented.
So try vmalloc when kmalloc failed.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>