The DMA mask must be set before, not after, the first DMA map operation, or
the first DMA map operation could in theory fail on some systems.

Fixes: b0eb57cb ("VMXNET3: Add support for virtual IOMMU")
Signed-off-by: Regis Duchesne <hpreg@vmware.com>
Acked-by: Ronak Doshi <doshir@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>