mm/lruvec: preemptively free dead folios during lru_add drain

Of all observable lruvec lock contention in our fleet, we find that ~24%
occurs when dead folios are present in lru_add batches at drain time. 
This is wasteful in the sense that the folio is added to the LRU just to
be immediately removed via folios_put_refs(), incurring two unnecessary
lock acquisitions.

Eliminate this overhead by preemptively cleaning up dead folios before
they make it into the LRU.  Use folio_ref_freeze() to filter folios whose
only remaining refcount is the batch ref.  When dead folios are found,
move them off the add batch and onto a temporary batch to be freed.

PG_active may be set on a batched folio as well as PG_unevictable (via
migration path).  Since filtered folios bypass the normal lru_add()
cleanup, both flags must be cleared before freeing.

During A/B testing on one of our prod instagram workloads (high-frequency
short-lived requests), the patch intercepted almost all dead folios before
they entered the LRU.  Data collected using the mm_lru_insertion
tracepoint shows the effectiveness of the patch:

Per-host LRU add averages at 95% CPU load
(60 hosts each side, 3 x 60s intervals)

            dead folios/min  total folios/min   dead %
unpatched:        1,297,785        19,341,986  6.7097%
patched:                 14        19,039,996  0.0001%

Within this workload, we save ~2.6M lock acquisitions per minute per host
as a result.

System-wide memory stats improved on the patched side also at 95% CPU load:
 - direct reclaim scanning reduced 7%
 - allocation stalls reduced 5.2%
 - compaction stalls reduced 12.3%
 - page frees reduced 4.9%

No regressions were observed in requests served per second or request tail
latency (p99).  Both metrics showed directional improvement at higher CPU
utilization (comparing 85% to 95%).

Note that tests were performed using classic LRU.

Link: https://lore.kernel.org/20260425053417.351146-1-jp.kobryn@linux.dev
Signed-off-by: JP Kobryn (Meta) <jp.kobryn@linux.dev>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <bhe@redhat.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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