x86/efi: Build our own page table structures

With commit e1a58320a38d ("x86/mm: Warn on W^X mappings") all users
booting on 64-bit UEFI machines see the following warning,

  ------------[ cut here ]------------
  WARNING: CPU: 7 PID: 1 at arch/x86/mm/dump_pagetables.c:225 note_page+0x5dc/0x780()
  x86/mm: Found insecure W+X mapping at address ffff88000005f000/0xffff88000005f000
  ...
  x86/mm: Checked W+X mappings: FAILED, 165660 W+X pages found.
  ...

This is caused by mapping EFI regions with RWX permissions. There
isn't much we can do to restrict the permissions for these regions due
to the way the firmware toolchains mix code and data, but we can at
least isolate these mappings so that they do not appear in the regular
kernel page tables.

In commit d2f7cbe7b26a ("x86/efi: Runtime services virtual mapping")
we started using 'trampoline_pgd' to map the EFI regions because there
was an existing identity mapping there which we use during the
SetVirtualAddressMap() call and for broken firmware that accesses
those addresses.

But 'trampoline_pgd' shares some PGD entries with 'swapper_pg_dir' and
does not provide the isolation we require. Notably the virtual address
for __START_KERNEL_map and MODULES_START are mapped by the same PGD
entry so we need to be more careful when copying changes over in
efi_sync_low_kernel_mappings().

This patch doesn't go the full mile, we still want to share some PGD
entries with 'swapper_pg_dir'. Having completely separate page tables
brings its own issues such as synchronising new mappings after memory
hotplug and module loading. Sharing also keeps memory usage down.

Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>