mm/page_ext: init page_ext early if there are no deferred struct pages

page_ext must be initialized after all struct pages are initialized. 
Therefore, page_ext is initialized after page_alloc_init_late(), and can
optionally be initialized earlier via early_page_ext kernel parameter
which as a side effect also disables deferred struct pages.

Allow to automatically init page_ext early when there are no deferred
struct pages in order to be able to use page_ext during kernel boot and
track for example page allocations early.

[pasha.tatashin@soleen.com: fix build with CONFIG_PAGE_EXTENSION=n]
  Link: https://lkml.kernel.org/r/20230118155251.2522985-1-pasha.tatashin@soleen.com
Link: https://lkml.kernel.org/r/20230117204617.1553748-1-pasha.tatashin@soleen.com


Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Charan Teja Kalla <quic_charante@quicinc.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Li Zhe <lizhe.67@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>