efi/unaccepted: Make sure unaccepted table is mapped

Unaccepted table is now allocated from EFI_ACPI_RECLAIM_MEMORY. It
translates into E820_TYPE_ACPI, which is not added to memblock and
therefore not mapped in the direct mapping.

This causes a crash on the first touch of the table.

Use memblock_add() to make sure that the table is mapped in direct
mapping.

Align the range to the nearest page borders. Ranges smaller than page
size are not mapped.

Fixes: e7761d82 ("efi/unaccepted: Use ACPI reclaim memory for unaccepted memory table")
Reported-by: Hongyu Ning <hongyu.ning@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>