mm: reuse_swap_page replaces can_share_swap_page

A good place to free up old swap is where do_wp_page(), or do_swap_page(),
is about to redirty the page: the data on disk is then stale and won't be
read again; and if we do decide to write the page out later, using the
previous swap location makes an unnecessary disk seek very likely.

So give can_share_swap_page() the side-effect of delete_from_swap_cache()
when it safely can.  And can_share_swap_page() was always a misleading
name, the more so if it has a side-effect: rename it reuse_swap_page().

Irrelevant cleanup nearby: remove swap_token_default_timeout definition
from swap.h: it's used nowhere.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Robin Holt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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