mm, memory_hotplug: __offline_pages fix wrong locking

Jan has noticed that we do double unlock on some failure paths when
offlining a page range. This is indeed the case when
test_pages_in_a_zone respp. start_isolate_page_range fail. This was an
omission when forward porting the debugging patch from an older kernel.

Fix the issue by dropping mem_hotplug_done from the failure condition
and keeping the single unlock in the catch all failure path.

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: 7960509329c2 ("mm, memory_hotplug: print reason for the offlining failure")
Signed-off-by: Michal Hocko <[e-mail]>
Reported-by: Jan Kara <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Tested-by: Jan Kara <[e-mail]>
Reviewed-by: Oscar Salvador <[e-mail]>
Reviewed-by: Andrew Morton <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memory_hotplug.c

···
 	   we assume this for now. .*/
 	if (!test_pages_in_a_zone(start_pfn, end_pfn, &valid_start,
 				  &valid_end)) {
-		mem_hotplug_done();
 		ret = -EINVAL;
 		reason = "multizone range";
 		goto failed_removal;
···
 				       MIGRATE_MOVABLE,
 				       SKIP_HWPOISON | REPORT_FAILURE);
 	if (ret) {
-		mem_hotplug_done();
 		reason = "failure to isolate range";
 		goto failed_removal;
 	}