Comment 7 for bug 1851380

I'm no expert in either kernel or C programming and I didn't test this patch, but it looks good to me.

Also, the resulting code is quite similar to the one Fedora is currently using - they accidentally fixed this bug in https://src.fedoraproject.org/rpms/kernel/c/5df4c5562f191b434edf57e8c92b176baad37cba.
(Debian also seems to be affected: https://salsa.debian.org/kernel-team/linux/blob/4dcfa860/debian/patches/features/all/lockdown/0002-Add-a-SysRq-option-to-lift-kernel-lockdown.patch)