x86: Make E820_X_MAX unconditionally larger than E820MAX

It's really not necessary to limit E820_X_MAX to 128 in the non-EFI
case.  This commit drops E820_X_MAX's dependency on CONFIG_EFI, so that
E820_X_MAX is always at least slightly larger than E820MAX.

The real motivation behind this is actually to prevent some issues in
the Xen kernel, where the XENMEM_machine_memory_map hypercall can
produce an e820 map larger than 128 entries, even on systems where the
original e820 table was quite a bit smaller than that, depending on how
many IOAPICs are installed on the system.

Signed-off-by: Alex Thorlton <athorlton@sgi.com>
Suggested-by: Ingo Molnar <mingo@redhat.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>