mm/page_poison.c: enable PAGE_POISONING as a separate option

Page poisoning is currently set up as a feature if architectures don't
have architecture debug page_alloc to allow unmapping of pages.  It has
uses apart from that though.  Clearing of the pages on free provides an
increase in security as it helps to limit the risk of information leaks.
Allow page poisoning to be enabled as a separate option independent of
kernel_map pages since the two features do separate work.  Because of
how hiberanation is implemented, the checks on alloc cannot occur if
hibernation is enabled.  The runtime alloc checks can also be enabled
with an option when !HIBERNATION.

Credit to Grsecurity/PaX team for inspiring this work

Signed-off-by: Laura Abbott <[email protected]>
Cc: Rafael J. Wysocki <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Mathias Krause <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Jianyu Zhan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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