efi/x86: Fix boot regression on systems with invalid memmap entries

In efi_clean_memmap(), we do a pass over the EFI memory map to remove
bogus entries that may be returned on certain systems.

This recent commit:

  1db91035 ("efi: Add tracking for dynamically allocated memmaps")

refactored this code to pass the input to efi_memmap_install() via a
temporary struct on the stack, which is populated using an initializer
which inadvertently defines the value of its size field in terms of its
desc_size field, which value cannot be relied upon yet in the initializer
itself.

Fix this by using efi.memmap.desc_size instead, which is where we get
the value for desc_size from in the first place.

Reported-by: Jörg Otte <jrg.otte@gmail.com>
Tested-by: Jörg Otte <jrg.otte@gmail.com>
Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-efi@vger.kernel.org
Cc: jrg.otte@gmail.com
Cc: torvalds@linux-foundation.org
Cc: mingo@kernel.org
Link: https://lore.kernel.org/r/20200201233304.18322-1-ardb@kernel.org