x86/mm: Unbreak modules that rely on external PAGE_KERNEL availability

Commit 7744ccdb ("x86/mm: Add Secure Memory Encryption (SME)
support") as a side-effect made PAGE_KERNEL all of a sudden unavailable
to modules which can't make use of EXPORT_SYMBOL_GPL() symbols.

This is because once SME is enabled, sme_me_mask (which is introduced as
EXPORT_SYMBOL_GPL) makes its way to PAGE_KERNEL through _PAGE_ENC,
causing imminent build failure for all the modules which make use of all
the EXPORT-SYMBOL()-exported API (such as vmap(), __vmalloc(),
remap_pfn_range(), ...).

Exporting (as EXPORT_SYMBOL()) interfaces (and having done so for ages)
that take pgprot_t argument, while making it impossible to -- all of a
sudden -- pass PAGE_KERNEL to it, feels rather incosistent.

Restore the original behavior and make it possible to pass PAGE_KERNEL
to all its EXPORT_SYMBOL() consumers.

[ This is all so not wonderful. We shouldn't need that "sme_me_mask"
  access at all in all those places that really don't care ab...