mm, swap: remove no longer needed _swap_info_get

mm, swap: remove no longer needed _swap_info_get

There are now only two users of _swap_info_get after consolidating these
callers, folio_free_swap and swp_swapcount.

folio_free_swap already holds the folio lock, and the folio must be in the
swap cache, _swap_info_get is redundant.

For swp_swapcount, it should use get_swap_device instead.  get_swap_device
increases the device ref count, which is actually a bit safer.  The only
current use is smap walking, and the performance change here is tiny.

And after these changes, _swap_info_get is no longer used, so we can
safely remove it.

Link: https://lkml.kernel.org/r/20251220-swap-table-p2-v5-19-8862a265a033@tencent.com
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Cc: Yosry Ahmed <yosry.ahmed@linux.dev>
Cc: Deepanshu Kartikey <kartikey406@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>