arm64: mte: Lock a page for MTE tag initialisation

Initialising the tags and setting PG_mte_tagged flag for a page can race
between multiple set_pte_at() on shared pages or setting the stage 2 pte
via user_mem_abort(). Introduce a new PG_mte_lock flag as PG_arch_3 and
set it before attempting page initialisation. Given that PG_mte_tagged
is never cleared for a page, consider setting this flag to mean page
unlocked and wait on this bit with acquire semantics if the page is
locked:

- try_page_mte_tagging() - lock the page for tagging, return true if it
  can be tagged, false if already tagged. No acquire semantics if it
  returns true (PG_mte_tagged not set) as there is no serialisation with
  a previous set_page_mte_tagged().

- set_page_mte_tagged() - set PG_mte_tagged with release semantics.

The two-bit locking is based on Peter Collingbourne's idea.

Signed-off-by: Catalin Marinas <[email protected]>
Signed-off-by: Peter Collingbourne <[email protected]>
Reviewed-by: Steven Price <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: Marc Zyngier <[email protected]>
Cc: Peter Collingbourne <[email protected]>
Reviewed-by: Cornelia Huck <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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