virtio_net: enable big packets for large MTU values

virtio_net: enable big packets for large MTU values

If one enables e.g. jumbo frames without mergeable
buffers, packets won't fit in 1500 byte buffers
we use. Switch to big packet mode instead.
TODO: make sizing more exact, possibly extend small
packet mode to use larger pages.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 2e123b44a3c19de75f40ee0081d6d4fc04adfdc7)

Orabug: 26584452

Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Conflicts:
	drivers/net/virtio_net.c
Due to the lack of commit d0c2c9973ecd ("net: use core MTU range
checking in virt drivers") the MTU size check is still done in the
virtio_net.