mm/swap_state.c: fix the obsolete code comment

mm/swap_state.c: fix the obsolete code comment

Patch series "Tiny cleanup and improvements about SWAP code".

These are all made during review and from reading the patchset "[PATCH v3
00/13] mm, swap: rework of swap allocator locks" from Kairui.

This patch (of 12):

Since commit 85a1333417a7 ("mm/swap: use dedicated entry for swap in
folio"), there's a dedicated field in folio for swap entry.  Let's update
the code comment above add_to_swap_cache() accordingly.

Link: https://lkml.kernel.org/r/20250205092721.9395-1-bhe@redhat.com
Link: https://lkml.kernel.org/r/20250205092721.9395-2-bhe@redhat.com
Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Kairui Song <kasong@tencent.com>
Cc: Baoquan he <bhe@redhat.com>
Cc: Chris Li <chrisl@kernel.org> (Google)
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>