mm/swapfile.c: update the code comment above swap_count_continued()

Now, swap_count_continued() has two callers, __swap_duplicate() and
__swap_entry_free_locked(), the relevant code comment is stale.  Update it
to reflect the current situation.

[bhe@redhat.com: v2]
  Link: https://lkml.kernel.org/r/Z6V0/UvG1fvkQ4t/@fedora
Link: https://lkml.kernel.org/r/20250205092721.9395-7-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <ryncsn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>