ipv6: sr: fix possible use-after-free and null-ptr-deref

The pernet operations structure for the subsystem must be registered
before registering the generic netlink family.

Fixes: 915d7e5e

 ("ipv6: sr: add code base for control plane support of SR-IPv6")
Signed-off-by: Vasiliy Kovalev <kovalev@altlinux.org>
Link: https://lore.kernel.org/r/20240215202717.29815-1-kovalev@altlinux.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>