UPSTREAM: nfs: pass the correct prototype to read_cache_page

Fix the callbacks NFS 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>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Change-Id: Ia245fc02d550d34d80f4e8ebdeb6b744115ffdb3
(cherry picked from commit a46126ccc77e764429d63bf958d117f607f4b6c6)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>