securityfs: do not depend on CONFIG_SECURITY

Add a new Kconfig option SECURITYFS which will build securityfs support
but does not require CONFIG_SECURITY.  The only current user of
securityfs does not depend on CONFIG_SECURITY and there is no reason the
full LSM needs to be built to build this fs.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>