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

commit 5dc64a3442b98eaa0e3730c35fcf00cf962a93e7 upstream.

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 2216d199b1430d1c0affb1498a9ebdbd9c0de439 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: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>