efi: Replace efi_memory_attributes_table_t 0-sized array with flexible array

While efi_memory_attributes_table_t::entry isn't used directly as an
array, it is used as a base for pointer arithmetic. The type is wrong
as it's not technically an array of efi_memory_desc_t's; they could be
larger. Regardless, leave the type unchanged and remove the old style
"0" array size. Additionally replace the open-coded entry offset code
with the existing efi_memdesc_ptr() helper.

Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>