to prevent wrongly overwriting fixmap that still want to use.

ACPI used to rely on low mappings being all linearly mapped and
grew a habit: it never really unmapped certain kinds of tables
after use.

This can cause problems - for example the hypothetical case
when some spurious access still references it.

v2: remove prev_map and prev_size in __apci_map_table
v3: let acpi_os_unmap_memory() call early_iounmap too, so remove extral calling to
early_acpi_os_unmap_memory
v4: fix typo in one acpi_get_table_with_size calling

Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Acked-by: Len Brown <len.brown@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>