x86/acpi: fix unmapping of low 1MB memory in acpi_os_unmap_memory

x86/acpi: fix unmapping of low 1MB memory in acpi_os_unmap_memory

Current code in acpi_os_map_memory uses the direct map in order to map memory
in the low 1MB, but acpi_os_unmap_memory doesn't takes that into account, and
always tries to perform a vunmap, which results in the following WARN:

(XEN) Xen WARN at vmap.c:185
(XEN) ----[ Xen-4.9-unstable  x86_64  debug=y   Tainted:  C   ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d0801369d7>] vmap.c#vm_free+0xd7/0xe0
[...]
(XEN) Xen call trace:
(XEN)    [<ffff82d0801369d7>] vmap.c#vm_free+0xd7/0xe0
(XEN)    [<ffff82d0802bdeda>] acpi_find_root_pointer+0x3a/0x170
(XEN)    [<ffff82d0802bd0ee>] acpi_os_get_root_pointer+0x4e/0x60
(XEN)    [<ffff82d0802d74d0>] domain_build.c#pvh_setup_acpi_xsdt+0x90/0x240
(XEN)    [<ffff82d0802d5d1a>] domain_build.c#pvh_setup_acpi+0x18a/0x2e0
(XEN)    [<ffff82d0802d3ad2>] domain_build.c#construct_dom0_pvh+0xd2/0x120
(XEN)    [<ffff82d0802c9174>] __start_xen+0x1d14/0x2420
(XEN)    [<ffff82d080100073>] __high_start+0x53/0x60

Fix this by checking if the virtual address passed to acpi_os_unmap_memory
belongs to the direct map.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>