FROMLIST: jffs2: pass the correct prototype to read_cache_page

Fix the callback jffs2 passes to read_cache_page to actually have the
proper type expected.  Casting around function pointers can easily
hide typing bugs, and defeats control flow protection.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Bug: 133186739
Change-Id: Ie5bfeff009a555c1894784458d2fe79a87f1be03
Link: https://lkml.org/lkml/2019/5/1/295


Signed-off-by: Sami Tolvanen <samitolvanen@google.com>