mm,swap: remove struct vma_swap_readahead

When VMA based swap readahead is introduced in commit ec560175 ("mm,
swap: VMA based swap readahead"), "struct vma_swap_readahead" is defined
to describe the readahead window.  Because we wanted to save the PTE
entries in the struct at that time.  But after commit 4f8fcf4c
("mm/swap: swap_vma_readahead() do the pte_offset_map()"), we no longer
save PTE entries in the struct.  The size of the struct becomes so small,
that it's better to use the fields of the struct directly.  This can
simplify the code to improve the code readability.  The line number of
source code reduces too.

No functionality change is expected in this patch.

Link: https://lkml.kernel.org/r/20240531081230.310128-3-ying.huang@intel.com


Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>