rcu: Mark rcu_state.gp_seq to detect more concurrent writes

The rcu_state structure's gp_seq field is only to be modified by the RCU
grace-period kthread, which is single-threaded.  This commit therefore
enlists KCSAN's help in enforcing this restriction.  This commit applies
KCSAN-specific primitives, so cannot go upstream until KCSAN does.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>