thanks for the detailed finding…
This patch was merged in mainline kernel in 4.3. So it is safe to use it, I believe. Note that there was another related patch in mainline (which should not impact you though):
b6dd8e0 arm64: errata: use KBUILD_CFLAGS_MODULE for erratum #843419
df057cc arm64: errata: add module build workaround for erratum #843419
Our current release is based on 4.2.4 kernel which has this fix already (it was backported to stable kernel), and our next release will be based on 4.3 which has it too.
And we are not planning any update on 15.09 release, nor are we planning any more Ubuntu releases.
Also for 15.09, the release was based on 4.2.0, not 4.2.4.