linux: enable MODULE_ALLOW_BTF_MISMATCH

Right now it looks like the BTFs are not reproducible between different builds
of the same kernel, and the kernel will refuse to load modules if the BTF
doesn't match. This can cause some interesting side effects when Nix
uses different substituters for different parts of the kernel.

This is far from ideal, and we _really_ should figure out how to actually
make the BTF building consistently reproducible, but that seems more
complicated, so maybe we should do this to get affected systems booting.

See also: https://lore.kernel.org/bpf/YfK18x%2FXrYL4Vw8o@syu-laptop/ ,
where the openSUSE people ran into similar issues.

pkgs/os-specific/linux/kernel/common-config.nix

@@ -45,6 +45,10 @@ let
       # aarch64 defconfig since 5.13
       DEBUG_INFO_REDUCED        = whenAtLeast "5.13" (option no);
       DEBUG_INFO_BTF            = whenAtLeast "5.2" (option yes);
+      # Allow loading modules with mismatched BTFs
+      # FIXME: figure out how to actually make BTFs reproducible instead
+      # See https://github.com/NixOS/nixpkgs/pull/181456 for details.
+      MODULE_ALLOW_BTF_MISMATCH = whenAtLeast "5.18" (option yes);
       BPF_LSM                   = whenAtLeast "5.7" (option yes);
       DEBUG_KERNEL              = yes;
       DEBUG_DEVRES              = no;