rcu: Use WRITE_ONCE() for ->rcu_need_heavy_qs

Currently raw_cpu_write() is used to clear the ->rcu_need_heavy_qs
field of the per-CPU rcu_data structure.  However, on x86 this is a
normal assignment, which does not play well with concurrent accesses.
This commit therefore upgrades the uses of raw_cpu_write() to its
concurrency-safe counterpart WRITE_ONCE() of an rdp pointer obtained
from this_cpu_ptr(&rcu_data).

KCSAN located this issue.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

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