net: Increase L2TPv3 buffer to fit jumboframes

Increase the allocated buffer size to fit larger packets.
Given that jumboframes can commonly be up to 9000 bytes the closest suitable
value seems to be 16 KiB.

Tested by running qemu towards a Linux L2TPv3 endpoint and pushing
jumboframe traffic through the interfaces.

Signed-off-by: Christian Svensson <blue@cmd.nu>
Signed-off-by: Jason Wang <jasowang@redhat.com>