sched: Assert for_each_thread() is properly locked

sched: Assert for_each_thread() is properly locked

list_for_each_entry_rcu() takes an optional fourth argument which
allows RCU to assert that the correct lock is held.  Several callers
of for_each_thread() rely on their caller to be holding the appropriate
lock, so this is a useful assertion to include.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Link: https://lore.kernel.org/r/20230821134428.2504912-1-willy@infradead.org