x86/mm/32: Fix W^X detection when page tables do not support NX

The x86 MM code now actively refuses to create writable+executable mappings,
and warns when there is an attempt to create one.

The 0day test robot ran across a warning triggered by module unloading on
32-bit kernels.  This was only seen on CPUs with NX support, but where a
32-bit kernel was built without PAE support.

On those systems, there is no room for the NX bit in the page
tables and _PAGE_NX is #defined to 0, breaking some of the W^X
detection logic in verify_rwx().  The X86_FEATURE_NX check in
there does not do any good here because the CPU itself supports
NX.

Fix it by checking for _PAGE_NX support directly instead of
checking CPU support for NX.

Note that since _PAGE_NX is actually defined to be 0 at
compile-time this fix should also end up letting the compiler
optimize away most of verify_rwx() on non-PAE kernels.

Fixes: 652c5bf380ad ("x86/mm: Refuse W^X violations")
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Acked-by: Peter Zijlstra (Intel) <[email protected]>
Link: https://lore.kernel.org/all/[email protected]/

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