Generate a warning if any W+X mappings are found at boot.

This is useful for discovering cases where the kernel is leaving
W+X mappings after applying NX, as such mappings are a security risk.

Look for a message in dmesg output like this:

x86/mm: Checked W+X mappings: passed, no W+X pages found.

or like this, if the check failed:

x86/mm: Checked W+X mappings: FAILED, <N> W+X pages found.

Note that even if the check fails, your kernel is possibly
still fine, as W+X mappings are not a security hole in
themselves, what they do is that they make the exploitation
of other unfixed kernel bugs easier.

There is no runtime or memory usage effect of this option
once the kernel has booted up - it's a one time check.

If in doubt, say "Y".