mm, page_owner: don't grab zone->lock for init_pages_in_zone()

mm, page_owner: don't grab zone->lock for init_pages_in_zone()

init_pages_in_zone() is run under zone->lock, which means a long lock
time and disabled interrupts on large machines.  This is currently not
an issue since it runs early in boot, but a later patch will change
that.

However, like other pfn scanners, we don't actually need zone->lock even
when other cpus are running.  The only potentially dangerous operation
here is reading bogus buddy page owner due to race, and we already know
how to handle that.  The worst that can happen is that we skip some
early allocated pages, which should not affect the debugging power of
page_owner noticeably.

Link: http://lkml.kernel.org/r/20170720134029.25268-4-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Yang Shi <yang.shi@linaro.org>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Vinayak Menon <vinmenon@codeaurora.org>
Cc: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>