Security: add static to security_ops and default_security_ops variable

Enhance the security framework to support resetting the active security
module. This eliminates the need for direct use of the security_ops and
default_security_ops variables outside of security.c, so make security_ops
and default_security_ops static. Also remove the secondary_ops variable as
a cleanup since there is no use for that. secondary_ops was originally used by
SELinux to call the "secondary" security module (capability or dummy),
but that was replaced by direct calls to capability and the only
remaining use is to save and restore the original security ops pointer
value if SELinux is disabled by early userspace based on /etc/selinux/config.
Further, if we support this directly in the security framework, then we can
just use &default_security_ops for this purpose since that is now available.

Signed-off-by: Zhitong Wang <zhitong.wangzt@alibaba-inc.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>