Revert selinux overlayfs mmap and mprotect access checks for kernel-6.12 and kernel-6.18

[Sparksssj]

Description of changes:

• Restore the selinux overlayfs mmap/mprotect revert (added in Bugfix/selinux #496, dropped in v8.0.0 Kernel update #526) for kernel-6.12 and kernel-6.18.
• Without it, file:execmod on overlayfs is checked against the mounter (runtime_t) instead of the process, so UPX-packed/JIT container binaries are denied and crash at startup.
  kernel-6.1 is unaffected.

Testing done:
Built aws-k8s-1.35 (6.12.100) and aws-k8s-1.36 (6.18.41) AMIs from this branch and ran the execmod reproducer as control_t on overlayfs under SELinux enforcing. On both kernels the previously-failing sequence now passes:

  === test_execmod: mmap(R|W, MAP_PRIVATE) -> write (COW) -> mprotect(R|X) ===
  mprotect(R|X) SUCCEEDED (no denial here)
  Exit 0, no `avc: denied` in dmesg.

Terms of contribution:

By submitting this pull request, I agree that this contribution is dual-licensed under the terms of both the Apache License, version 2.0, and the MIT license.

[darsh-shah-sh]

@Sparksssj Thanks for putting the fix. Do you know what the timeline on releasing this fix is?