UPSTREAM: xfrm: Make function xfrmi_get_link_net() static

Fixes the following sparse warning:

net/xfrm/xfrm_interface.c:745:12: warning:
 symbol 'xfrmi_get_link_net' was not declared. Should it be static?

Fixes: f203b76d7809 ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
(cherry picked from commit 211d6f2dc883fe2235532d7ec4ed7e8222957ae0)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I71d25b41c0e87945281c6c8dedf65953a4cef4dc