rcu: Make srcu_read_lock_held() call common lockdep-enabled function

A common debug_lockdep_rcu_enabled() function is used to check whether
RCU lockdep splats should be reported, but srcu_read_lock() does not
use it.  This commit therefore brings srcu_read_lock_held() up to date.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>