mm/page_ext: do not allocate space for page_ext->flags if not needed

mm/page_ext: do not allocate space for page_ext->flags if not needed

There is 8 byte page_ext->flags field allocated per page whenever
CONFIG_PAGE_EXTENSION is enabled.  However, not every user of page_ext
uses flags.  Therefore, check whether flags is needed at least by one user
and if so allocate space for it.

For example when page_table_check is enabled, on a machine with 128G
of memory before the fix:

[    2.244288] allocated 536870912 bytes of page_ext
after the fix:
[    2.160154] allocated 268435456 bytes of page_ext

Also, add a kernel-doc comment before page_ext_operations that describes
the fields, and remove check if need() is set, as that is now a required
field.

[[email protected]: address comments from Mike Rapoport]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Pasha Tatashin <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Acked-by: David Rientjes <[email protected]>
Reviewed-by: Mike Rapoport (IBM) <[email protected]>
Cc: Charan Teja Kalla <[email protected]>
Cc: Li Zhe <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>