rcutorture: Make srcu_lockdep.sh check kernel Kconfig

The srcu_lockdep.sh currently blindly trusts the rcutorture SRCU-P
scenario to build its kernel with lockdep enabled.  Of course, this
dependency might not be obvious to someone rebalancing SRCU scenarios.
This commit therefore adds code to srcu_lockdep.sh that verifies that
the .config file has lockdep enabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>