modules: mark ref_module static

commit 7ef5264de773279b9f23b6cc8afb5addb30e970b upstream.

ref_module isn't used anywhere outside of module.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>