mm: zswap: rename is_zswap_enabled() to zswap_is_enabled()

In preparation for introducing a similar function, rename
is_zswap_enabled() to use zswap_* prefix like other zswap functions.

Link: https://lkml.kernel.org/r/20240611024516.1375191-1-yosryahmed@google.com


Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Barry Song <baohua@kernel.org>
Reviewed-by: Nhat Pham <nphamcs@gmail.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>