bcachefs: Btree key cache shrinker fix

bcachefs: Btree key cache shrinker fix

The shrinker assumes freed key cache items are ordered by age, so that
it doesn't have to scan the full list to find items that are old enough
(according to the srcu code) to be freed.

But percpu freelists broke this ordering; this patch fixes this by
ensuring we insert items into the proper position.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>