memory_hotplug: fix kernel_panic on offline page processing

Within show_valid_zones() the function test_pages_in_a_zone() should be
called for online memory blocks only.

Otherwise it might lead to the VM_BUG_ON due to uninitialized struct
pages (when CONFIG_DEBUG_VM_PGFLAGS kernel option is set):

 page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
 ------------[ cut here ]------------
 Call Trace:
 ([<000000000038f91e>] test_pages_in_a_zone+0xe6/0x168)
  [<0000000000923472>] show_valid_zones+0x5a/0x1a8
  [<0000000000900284>] dev_attr_show+0x3c/0x78
  [<000000000046f6f0>] sysfs_kf_seq_show+0xd0/0x150
  [<00000000003ef662>] seq_read+0x212/0x4b8
  [<00000000003bf202>] __vfs_read+0x3a/0x178
  [<00000000003bf3ca>] vfs_read+0x8a/0x148
  [<00000000003bfa3a>] ksys_read+0x62/0xb8
  [<0000000000bc2220>] system_call+0xdc/0x2d8

That VM_BUG_ON was triggered by the page poisoning introduced in
mm/sparse.c with the git commit d0dc12e8 ("mm/memory_hotplug:
optimize memory hotplug").

With the same commit the new 'nid' field has been added to the struct
memory_block in order to store and later on derive the node id for
offline pages (instead of accessing struct page which might be
uninitialized).  But one reference to nid in show_valid_zones() function
has been overlooked.  Fixed with current commit.  Also, nr_pages will
not be used any more after test_pages_in_a_zone() call, do not update
it.

Link: http://lkml.kernel.org/r/20180828090539.41491-1-zaslonko@linux.ibm.com
Fixes: d0dc12e8

 ("mm/memory_hotplug: optimize memory hotplug")
Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Pavel Tatashin <pavel.tatashin@microsoft.com>
Cc: <stable@vger.kernel.org>	[4.17+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>