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.

Link: http://lkml.kernel.org/r/20190520055731.24538-4-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/jffs2/fs.c

@@ -687,7 +687,7 @@ unsigned char *jffs2_gc_fetch_page(struct jffs2_sb_info *c,
 	struct page *pg;
 
 	pg = read_cache_page(inode->i_mapping, offset >> PAGE_SHIFT,
-			     (void *)jffs2_do_readpage_unlock, inode);
+			     jffs2_do_readpage_unlock, inode);
 	if (IS_ERR(pg))
 		return (void *)pg;