FROMGIT: BACKPORT: vsock/virtio: Rename virtio_vsock_alloc_skb()

In preparation for nonlinear allocations for large SKBs, rename
virtio_vsock_alloc_skb() to virtio_vsock_alloc_linear_skb() to indicate
that it returns linear SKBs unconditionally and switch all callers over
to this new interface for now.

No functional change.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Will Deacon <will@kernel.org>
Message-Id: <20250717090116.11987-6-will@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit a9d5b4a3ae3d72d8d8a74335aa06ff2e53082613
 git://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git

 vhost)
Bug: 421244320
[willdeacon@: Fix context conflict in virtio_transport_common.c due to
 missing zero-copy support]
Signed-off-by: Will Deacon <willdeacon@google.com>
Change-Id: I1d0c1b18e7fd9dab855981a331ee0cb5f46de675