mm: zswap: make same_filled functions folio-friendly

mm: zswap: make same_filled functions folio-friendly

A variable name 'page' is used in zswap_is_folio_same_filled() and
zswap_fill_page() to point at the kmapped data in a folio. Use 'data'
instead to avoid confusion and stop it from showing up when searching
for 'page' references in mm/zswap.c.

While we are at it, move the kmap/kunmap calls into zswap_fill_page(),
make it take in a folio, and rename it to zswap_fill_folio().

Link: https://lkml.kernel.org/r/20240524033819.1953587-4-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>