workqueue: Use variable name irq_flags for saving local irq flags

Using the generic term `flags` for irq flags is conventional but can be
confusing as there's quite a bit of code dealing with work flags which
involves some subtleties. Let's use a more explicit name `irq_flags` for
local irq flags. No functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Lai Jiangshan <jiangshanlai@gmail.com>