Revert "x86/mm: Expand static page table for fixmap space"

This reverts commit 3a8304b7, which was
upstream commit 05ab1d8a4b36ee912b7087c6da127439ed0a903e.

Ben Hutchings writes:

This backport is incorrect.  The part that updated __startup_64() in
arch/x86/kernel/head64.c was dropped, presumably because that function
doesn't exist in 4.9.  However that seems to be an essential of the
fix.  In 4.9 the startup_64 routine in arch/x86/kernel/head_64.S would
need to be changed instead.

I also found that this introduces new boot-time warnings on some
systems if CONFIG_DEBUG_WX is enabled.

So, unless someone provides fixes for those issues, I think this should
be reverted for the 4.9 branch.

Signed-off-by: Sasha Levin <sashal@kernel.org>