UPSTREAM: modules: mark ref_module static

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

Bug: 157965270
Bug: 171277690
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jessica Yu <jeyu@kernel.org>
(cherry picked from commit 7ef5264de773279b9f23b6cc8afb5addb30e970b)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I68f39675f53b745664c833ac150d838563c78aae
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>