mm/swap: rename swap_is_has_cache() to swap_only_has_cache()

There are two predicates in the name of swap_is_has_cache() which is
confusing.  Renaming it to remove the confusion and can better reflect its
functionality.

Link: https://lkml.kernel.org/r/20250205092721.9395-6-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>