rculist: Add ASSERT_EXCLUSIVE_ACCESS() to __list_splice_init_rcu()

After the sync() in __list_splice_init_rcu(), there should be no
readers traversing the old list.  This commit therefore enlists the
help of KCSAN to verify this condition via a pair of calls to
ASSERT_EXCLUSIVE_ACCESS().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Marco Elver <elver@google.com>