stop_machine: Use {READ,WRITE)_ONCE() for multi_cpu_stop() ->state

The multi_stop_data structure's ->state field is updated and read
concurrently, so this commit replaces the current C-language accesses
with READ_ONCE() and WRITE_ONCE().

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
1 file changed