module: Fix NULL vs IS_ERR checking for module_get_next_page

The module_get_next_page() function return error pointers on error
instead of NULL.
Use IS_ERR() to check the return value to fix this.

Fixes: b1ae6dc4

 ("module: add in-kernel support for decompressing")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>