sf00204509: rcu lock fix backport

srcu: Lock srcu_data structure in srcu_gp_start()
The srcu_gp_start() function is called with the srcu_struct structure's
->lock held, but not with the srcu_data structure's ->lock. This is
problematic because this function accesses and updates the srcu_data
structure's ->srcu_cblist, which is protected by that lock. Failing to
hold this lock can result in corruption of the SRCU callback lists,
which in turn can result in arbitrarily bad results.

This commit therefore makes srcu_gp_start() acquire the srcu_data
structure's ->lock across the calls to rcu_segcblist_advance() and
rcu_segcblist_accelerate(), thus preventing this corruption.

Reported-by: Bart Van Assche <email address hidden>
Reported-by: Christoph Hellwig <email address hidden>
Reported-by: Sebastian Kuzminsky <email address hidden>
Signed-off-by: Dennis Krein <email address hidden>
Signed-off-by: Paul E. McKenney <email address hidden>
Tested-by: Dennis Krein <email address hidden>
Cc: <email address hidden> # 4.12.x

f5f464012e3f srcu: Lock srcu_data structure in srcu_gp_start()
9a0941dc9e50 srcu: Prohibit call_srcu() use under raw spinlocks