Fix incompatible pointer type warning

This warning was accidentally introduced by commit
f3ab88d646 which updated the
.readpages() implementation.  The fix is to simply cast
the helper function to the appropriate type when passed.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zpl_file.c

@@ -305,7 +305,8 @@ static int
 zpl_readpages(struct file *filp, struct address_space *mapping,
 	struct list_head *pages, unsigned nr_pages)
 {
-	return (read_cache_pages(mapping, pages, zpl_readpage, filp));
+	return (read_cache_pages(mapping, pages,
+	    (filler_t *)zpl_readpage, filp));
 }
 
 int