ACPI / APEI: Replace ioremap_page_range() with fixmap

Replace ghes_io{re,un}map_pfn_{nmi,irq}()s use of ioremap_page_range()
with __set_fixmap() as ioremap_page_range() may sleep to allocate a new
level of page-table, even if its passed an existing final-address to
use in the mapping.

clear_fixmap() does the TLB invalidation on arm64, but not on x86,
so arch_apei_flush_tlb_one() gets to live another day.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: James Morse <james.morse@arm.com>