virtio_net: enable premapped mode for merge and small by default

Currently, the virtio core will perform a dma operation for each
buffer. Although, the same page may be operated multiple times.

In premapped mod, we can perform only one dma operation for the pages of
the alloc frag. This is beneficial for the iommu device.

kernel command line: intel_iommu=on iommu.passthrough=0

       |  strict=0  | strict=1
Before |  775496pps | 428614pps
After  | 1109316pps | 742853pps

In the 6.11, we disabled this feature because a regress [1].

Now, we fix the problem and re-enable it.

[1]: http://lore.kernel.org/all/8b20cc28-45a9-4643-8e87-ba164a540c0a@oracle.com



Tested-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>