virtio-net: use mtu size as buffer length for big packets

Currently add_recvbuf_big() allocates MAX_SKB_FRAGS segments for big
packets even when GUEST_* offloads are not present on the device.
However, if guest GSO is not supported, it would be sufficient to
allocate segments to cover just up the MTU size and no further.
Allocating the maximum amount of segments results in a large waste of
buffer space in the queue, which limits the number of packets that can
be buffered and can result in reduced performance.

Therefore, if guest GSO is not supported, use the MTU to calculate the
optimal amount of segments required.

Below is the iperf TCP test results over a Mellanox NIC, using vDPA for
1 VQ, queue size 1024, before and after the change, with the iperf
server running over the virtio-net interface.

MTU(Bytes)/Bandwidth (Gbit/s)
             Before   After
  1500        22.5     22.4
  9000        12.8     25.9

And result of queue size 256.
MTU(Bytes)/Bandwidth (Gbit/s)
             Before   After
  9000        2.15     11.9

With this patch no degradation is observed with multiple below tests and
feature bit combinations. Results are summarized below for q depth of
1024. Interface MTU is 1500 if MTU feature is disabled. MTU is set to 9000
in other tests.

Features/              Bandwidth (Gbit/s)
                         Before   After
mtu off                   20.1     20.2
mtu/indirect on           17.4     17.3
mtu/indirect/packed on    17.2     17.2

Signed-off-by: Gavin Li <gavinl@nvidia.com>
Reviewed-by: Gavi Teitz <gavi@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Message-Id: <20220914144911.56422-3-gavinl@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>