FROMGIT: BACKPORT: vsock/virtio: Allocate nonlinear SKBs for handling large transmit buffers

When transmitting a vsock packet, virtio_transport_send_pkt_info() calls
virtio_transport_alloc_linear_skb() to allocate and fill SKBs with the
transmit data. Unfortunately, these are always linear allocations and
can therefore result in significant pressure on kmalloc() considering
that the maximum packet size (VIRTIO_VSOCK_MAX_PKT_BUF_SIZE +
VIRTIO_VSOCK_SKB_HEADROOM) is a little over 64KiB, resulting in a 128KiB
allocation for each packet.

Rework the vsock SKB allocation so that, for sizes with page order
greater than PAGE_ALLOC_COSTLY_ORDER, a nonlinear SKB is allocated
instead with the packet header in the SKB and the transmit data in the
fragments. Note that this affects both the vhost and virtio transports.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
Message-Id: <20250717090116.11987-10-will@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit c7991b44d7b44f9270dec63acd0b2965d29aab43
 git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git

 vhost)
Bug: 421244320
Bug: 436528862
[willdeacon@: Inline skb_copy_datagram_from_iter() call due to missing
 zero-copy support and therefore missing definition of
 virtio_transport_fill_skb()]
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: Icf2d6179ca7ac36383604ba6e0fd96a7f5a9c767