workqueue: rename @id to @pi in for_each_each_pool()

Rename @id argument of for_each_pool() to @pi so that it doesn't get
reused accidentally when for_each_pool() is used in combination with
other iterators.

This patch is purely cosmetic.

Signed-off-by: Tejun Heo <tj@kernel.org>

diff --cc kernel/workqueue.c
Simple merge