x86/xen: dont add memory above max allowed allocation

Don't allow memory to be added above the allowed maximum allocation
limit set by Xen.

Trying to do so would result in cases like the following:

[  584.559652] ------------[ cut here ]------------
[  584.564897] WARNING: CPU: 2 PID: 1 at ../arch/x86/xen/multicalls.c:129 xen_alloc_pte+0x1c7/0x390()
[  584.575151] Modules linked in:
[  584.578643] Supported: Yes
[  584.581750] CPU: 2 PID: 1 Comm: swapper/0 Not tainted 4.4.120-92.70-default #1
[  584.590000] Hardware name: Cisco Systems Inc UCSC-C460-M4/UCSC-C460-M4, BIOS C460M4.4.0.1b.0.0629181419 06/29/2018
[  584.601862]  0000000000000000 ffffffff813175a0 0000000000000000 ffffffff8184777c
[  584.610200]  ffffffff8107f4e1 ffff880487eb7000 ffff8801862b79c0 ffff88048608d290
[  584.618537]  0000000000487eb7 ffffea0000000201 ffffffff81009de7 ffffffff81068561
[  584.626876] Call Trace:
[  584.629699]  [<ffffffff81019ad9>] dump_trace+0x59/0x340
[  584.635645]  [<ffffffff81019eaa>] show_stack_log_lvl+0xea/0x170
[  584.642391]  [<ffffffff8101ac51>] show_stack+0x21/0x40
[  584.648238]  [<ffffffff813175a0>] dump_stack+0x5c/0x7c
[  584.654085]  [<ffffffff8107f4e1>] warn_slowpath_common+0x81/0xb0
[  584.660932]  [<ffffffff81009de7>] xen_alloc_pte+0x1c7/0x390
[  584.667289]  [<ffffffff810647f0>] pmd_populate_kernel.constprop.6+0x40/0x80
[  584.675241]  [<ffffffff815ecfe8>] phys_pmd_init+0x210/0x255
[  584.681587]  [<ffffffff815ed207>] phys_pud_init+0x1da/0x247
[  584.687931]  [<ffffffff815edb3b>] kernel_physical_mapping_init+0xf5/0x1d4
[  584.695682]  [<ffffffff815e9bdd>] init_memory_mapping+0x18d/0x380
[  584.702631]  [<ffffffff81064699>] arch_add_memory+0x59/0xf0

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>