vhost/vsock: Use kvmalloc/kvfree for larger packets.

When copying a large file over sftp over vsock, data size is usually 32kB,
and kmalloc seems to fail to try to allocate 32 32kB regions.

 vhost-5837: page allocation failure: order:4, mode:0x24040c0
 Call Trace:
  [<ffffffffb6a0df64>] dump_stack+0x97/0xdb
  [<ffffffffb68d6aed>] warn_alloc_failed+0x10f/0x138
  [<ffffffffb68d868a>] ? __alloc_pages_direct_compact+0x38/0xc8
  [<ffffffffb664619f>] __alloc_pages_nodemask+0x84c/0x90d
  [<ffffffffb6646e56>] alloc_kmem_pages+0x17/0x19
  [<ffffffffb6653a26>] kmalloc_order_trace+0x2b/0xdb
  [<ffffffffb66682f3>] __kmalloc+0x177/0x1f7
  [<ffffffffb66e0d94>] ? copy_from_iter+0x8d/0x31d
  [<ffffffffc0689ab7>] vhost_vsock_handle_tx_kick+0x1fa/0x301 [vhost_vsock]
  [<ffffffffc06828d9>] vhost_worker+0xf7/0x157 [vhost]
  [<ffffffffb683ddce>] kthread+0xfd/0x105
  [<ffffffffc06827e2>] ? vhost_dev_set_owner+0x22e/0x22e [vhost]
  [<ffffffffb683dcd1>] ? flush_kthread_worker+0xf3/0xf3
  [<ffffffffb6eb332e>] ret_from_fork+0x4e/0x80
  [<ffffffffb683dcd1>] ? flush_kthread_worker+0xf3/0xf3

Work around by doing kvmalloc instead.

Fixes: 433fc58e

 ("VSOCK: Introduce vhost_vsock.ko")
Signed-off-by: Junichi Uekawa <uekawa@chromium.org>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://lore.kernel.org/r/20220928064538.667678-1-uekawa@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>