I can confirm I am able to establish a L2TP/IPsec connection with xl2tpd_1.3.10-1+lp1760796_amd64.deb test package with Bionic's latest 4.15 kernel.
I'll need to bring up a VM for xenial, but happy to test with kernel 4.4 and 4.15 on xenial for any backport. The version of xl2tpd in xenial updates is currently 1.3.6+dfsg-4ubuntu0.16.04.1 and suspect the patch should apply cleanly to it.
I can confirm I am able to establish a L2TP/IPsec connection with xl2tpd_ 1.3.10- 1+lp1760796_ amd64.deb test package with Bionic's latest 4.15 kernel.
I'll need to bring up a VM for xenial, but happy to test with kernel 4.4 and 4.15 on xenial for any backport. The version of xl2tpd in xenial updates is currently 1.3.6+dfsg- 4ubuntu0. 16.04.1 and suspect the patch should apply cleanly to it.