tomoyo: use DEFINE_SRCU() to define tomoyo_ss

DEFINE_STATIC_SRCU() defines srcu struct and do init at build time.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <james.l.morris@oracle.com>