fix a hyperv W^X violation and remove vmalloc_exec

[Christoph Hellwig]

Hi all,

Dexuan reported a W^X violation due to the fact that the hyper hypercall
page due switching it to be allocated using vmalloc_exec.  The problem
is that PAGE_KERNEL_EXEC as used by vmalloc_exec actually sets writable
permissions in the pte.  This series fixes the issue by switching to the
low-level __vmalloc_node_range interface that allows specifing more
detailed permissions instead.  It then also open codes the other two
callers and removes the somewhat confusing vmalloc_exec interface.

Peter noted that the hyper hypercall page allocation also has another
long standing issue in that it shouldn't use the full vmalloc but just
the module space.  This issue is so far theoretical as the allocation is
done early in the boot process.  I plan to fix it with another bigger
series for 5.9.