jffs2: Use a folio in jffs2_garbage_collect_dnode()

jffs2: Use a folio in jffs2_garbage_collect_dnode()

Call read_cache_folio() instead of read_cache_page() to get the folio
containing the page.  No attempt is made here to support large folios
as I assume that will never be interesting for jffs2.  Includes a switch
from kmap to kmap_local which looks safe.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://lore.kernel.org/r/20240814195915.249871-3-willy@infradead.org
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>