rcu: Move rcu_tasks_trace_srcu_struct out of #ifdef CONFIG_TASKS_RCU_GENERIC

rcu: Move rcu_tasks_trace_srcu_struct out of #ifdef CONFIG_TASKS_RCU_GENERIC

Moving the rcu_tasks_trace_srcu_struct structure instance out
from under the CONFIG_TASKS_RCU_GENERIC Kconfig option permits
the CONFIG_TASKS_TRACE_RCU Kconfig option to stop enabling this
CONFIG_TASKS_RCU_GENERIC Kconfig option.  This commit also therefore
makes it so.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>