mm/memory_hotplug.c: update start_pfn in zone and pg_data when spanned_pages == 0.

mm/memory_hotplug.c: update start_pfn in zone and pg_data when spanned_pages == 0.

If we hot-remove memory only and leave the cpus alive, the corresponding
node will not be removed.  But the node_start_pfn and node_spanned_pages
in pg_data will be reset to 0.  In this case, when we hot-add the memory
back next time, the node_start_pfn will always be 0 because no pfn is less
than 0.  After that, if we hot-remove the memory again, it will cause
kernel panic in function find_biggest_section_pfn() when it tries to scan
all the pfns.

The zone will also have the same problem.

This patch sets start_pfn to the start_pfn of the section being added when
spanned_pages of the zone or pg_data is 0.

---How to reproduce---

1. hot-add a container with some memory and cpus;
2. hot-remove the container's memory, and leave cpus there;
3. hot-add these memory again;
4. hot-remove them again;

then, the kernel will panic.

---Call trace---

[10530.646285] BUG: unable to handle kernel paging request at
00000fff82a8cc38
[10530.729670] IP: [<ffffffff811c0d55>] find_biggest_section_pfn+0xe5/0x180
......
[10533.064975] Call Trace:
[10533.094162]  [<ffffffff811c0fcf>] ? __remove_zone+0x2f/0x1b0
[10533.161757]  [<ffffffff811c1124>] __remove_zone+0x184/0x1b0
[10533.228318]  [<ffffffff811c11dc>] __remove_section+0x8c/0xb0
[10533.295916]  [<ffffffff811c12e7>] __remove_pages+0xe7/0x120
[10533.362476]  [<ffffffff81654f7c>] arch_remove_memory+0x2c/0x80
[10533.432151]  [<ffffffff81655bb6>] remove_memory+0x56/0x90
[10533.496633]  [<ffffffff813da0c8>]
acpi_memory_device_remove_memory+0x48/0x73
[10533.580846]  [<ffffffff813da55a>] acpi_memory_device_notify+0x153/0x274
[10533.659865]  [<ffffffff813a63cf>] ? acpi_bus_get_device+0x2f/0x77
[10533.732653]  [<ffffffff813a6589>] ? acpi_bus_notify+0xb5/0xec
[10533.801291]  [<ffffffff813b6786>] acpi_ev_notify_dispatch+0x41/0x5f
[10533.876156]  [<ffffffff813a3867>] acpi_os_execute_deferred+0x27/0x34
[10533.952062]  [<ffffffff81090589>] process_one_work+0x219/0x680
[10534.021736]  [<ffffffff81090528>] ? process_one_work+0x1b8/0x680
[10534.093488]  [<ffffffff813a3840>] ?
acpi_os_wait_events_complete+0x23/0x23
[10534.175622]  [<ffffffff810923be>] worker_thread+0x12e/0x320
[10534.242181]  [<ffffffff81092290>] ? manage_workers+0x110/0x110
[10534.311855]  [<ffffffff81098396>] kthread+0xc6/0xd0
[10534.370111]  [<ffffffff8167c7c4>] kernel_thread_helper+0x4/0x10
[10534.440824]  [<ffffffff81672230>] ? retint_restore_args+0x13/0x13
[10534.513612]  [<ffffffff810982d0>] ? __init_kthread_worker+0x70/0x70
[10534.588480]  [<ffffffff8167c7c0>] ? gs_change+0x13/0x13
......
[10535.045543] ---[ end trace 96d845dbf33fee11 ]---

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>