mm: swap.h: Remove deleted field from comments

The comment for struct swap_info_struct.lock incorrectly mentions fields
that have already been deleted from the structure.

Update the comments to accurately reflect the current struct
swap_info_struct.

There is no functional change.

Link: https://lkml.kernel.org/r/20250812-swap-scan-list-v3-2-6d73504d267b@kernel.org


Signed-off-by: Chris Li <chrisl@kernel.org>
Reviewed-by: Kairui Song <kasong@tencent.com>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Cc: Baoquan He <bhe@redhat.com>
Cc: "Huang, Ying" <ying.huang@linux.alibaba.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>