lib/list_sort: remove dummy cmp() calls to speed up merge_final()

lib/list_sort: remove dummy cmp() calls to speed up merge_final()

Historically, list_sort() implemented a hack in merge_final():
    if (unlikely(!++count))
        cmp(priv, b, b);

This was introduced 16 years ago in commit 835cc0c8477f ("lib: more
scalable list_sort()") so that callers could periodically invoke
cond_resched() within their comparison functions when merging highly
unbalanced lists.

An audit of the kernel tree reveals that fs/ubifs/ was the sole user of
this mechanism.  Recent discussions and inspections by Richard Weinberger
confirm that UBIFS lists are strictly bounded in size (a few thousand
elements at most), meaning it does not strictly rely on these dummy
callbacks to prevent soft lockups.

For the vast majority of list_sort() users (such as block layer IO
schedulers and file systems), this hack results in completely wasted
function calls.  In the worst-case scenario (merging an already sorted
list where 'a' is exhausted quickly), it results in approximately
(N/2)/256 unnecessary cmp() invocations.

Remove the dummy cmp(priv, b, b) fallback from merge_final().  This saves
unnecessary function calls, avoids branching overhead in the tight loop,
and slightly speeds up the final merge step for all generic list_sort()
users.

[akpm@linux-foundation.org: remove now-unused local]
Link: https://lkml.kernel.org/r/20260320180938.1827148-3-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Mars Cheng <marscheng@google.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Yu-Chun Lin <eleanor15x@gmail.com>
Cc: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>