grsecurity module: force a known good kernel package set

Previously, we would only set a default value, on the theory that
`boot.kernelPackages` could be used to sanely configure a custom grsec
kernel.  Regrettably, this is not the case and users who expect e.g.,
`boot.kernelPackages = pkgs.linuxPackages_latest` to work will end up
with a non-grsec kernel (this problem has come up twice on the bug
tracker recently).

With this patch, `security.grsecurity.enable = true` implies
`boot.kernelPackages = linuxPackages_grsec_nixos` and any customization
must be done via package override or by eschewing the module.