efi/x86: Clean up efi_systab_init() routine for legibility

Clean up the efi_systab_init() routine which maps the EFI system
table and copies the relevant pieces of data out of it.

The current routine is very difficult to read, so let's clean that
up. Also, switch to a R/O mapping of the system table since that is
all we need.

Finally, use a plain u64 variable to record the physical address of
the system table instead of pointlessly stashing it in a struct efi
that is never used for anything else.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Matthew Garrett <mjg59@google.com>
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/20200103113953.9571-13-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>