DPDK ICE driver interprets MTU as max frame size

When `lib/net.c:check_if_mtu()` sets `port_conf->rxmode.mtu`, DPDK's ICE driver interprets it as max frame size. This has been observed when the ICE driver uses the vetorial receiving path. This bug may also happen in the non-vetorial path, but has not been tested. A workaround consists of adding the Ethernet header and the VLAN header to the MTU parameter.

When the ICE driver uses the vetorial receiving path and the MTU is 2048 (the typical value in Gatekeeper's back interface), without the workaround, frames greater than 2048 are broken into two packets: the first packet has the first 2048 bytes, and the second packet has the remaining bytes.

The ICE driver has been heavily rewritten since DPDK 23.11, the version of DPDK that Gatekeeper v1.2.0 is based on. Therefore, this bug may not be present in newer versions of DPDK. The test plan [106. MTU Check Tests](https://doc.dpdk.org/dts/test_plans/mtu_update_test_plan.html) may prove helpful to test new DPDK versions and to report this bug, if it is still present.