virtio_net: merge dma operations when filling mergeable buffers

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

This patch, the driver does the dma operation and manages the dma
address based the feature premapped of virtio core.

This way, 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

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Message-Id: <20230810123057.43407-13-xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>