xen: do not reserve 2 pages of padding between hypervisor and fixmap.

When reserving space for the hypervisor the Xen paravirt backend adds
an extra two pages (this was carried forward from the 2.6.18-xen tree
which had them "for safety"). Depending on various CONFIG options this
can cause the boot time fixmaps to span multiple PMDs which is not
supported and triggers a WARN in early_ioremap_init().

This was exposed by 2216d199 which
moved the dmi table parsing earlier.
    x86: fix CONFIG_X86_RESERVE_LOW_64K=y

    The bad_bios_dmi_table() quirk never triggered because we do DMI setup
    too late. Move it a bit earlier.

There is no real reason to reserve these two extra pages and the
fixmap already incorporates FIX_HOLE which serves the same
purpose. None of the other callers of reserve_top_address do this.
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/enlighten.c

@@ -1370,7 +1370,7 @@ static void __init xen_reserve_top(void)
 	if (HYPERVISOR_xen_version(XENVER_platform_parameters, &pp) == 0)
 		top = pp.virt_start;
 
-	reserve_top_address(-top + 2 * PAGE_SIZE);
+	reserve_top_address(-top);
 #endif	/* CONFIG_X86_32 */
 }