Merge pull request #12574 from yuwata/network-mtu-issue-12552

network: do not always bump MTU with additional 4bytes

diff --cc src/network/networkd-link.c
Simple merge

diff --cc src/network/networkd-network.c
Simple merge

diff --cc src/network/networkd-network.h
Simple merge

diff --cc test/test-network/systemd-networkd-tests.py
Simple merge