FROMLIST: 9p: pass the correct prototype to read_cache_page

Fix the callback 9p 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>
Change-Id: I3738b7c0c7f77f85cd7007141400c36516dac6b8
Bug: 133186739
Link: https://lkml.org/lkml/2019/5/1/374


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