workqueue, irq_work: Build fix for !CONFIG_IRQ_WORK

workqueue, irq_work: Build fix for !CONFIG_IRQ_WORK

2f34d7337d98 ("workqueue: Fix queue_work_on() with BH workqueues") added
irq_work usage to workqueue; however, it turns out irq_work is actually
optional and the change breaks build on configuration which doesn't have
CONFIG_IRQ_WORK enabled.

Fix build by making workqueue use irq_work only when CONFIG_SMP and enabling
CONFIG_IRQ_WORK when CONFIG_SMP is set. It's reasonable to argue that it may
be better to just always enable it. However, this still saves a small bit of
memory for tiny UP configs and also the least amount of change, so, for now,
let's keep it conditional.

Verified to do the right thing for x86_64 allnoconfig and defconfig, and
aarch64 allnoconfig, allnoconfig + prink disable (SMP but nothing selects
IRQ_WORK) and a modified aarch64 Kconfig where !SMP and nothing selects
IRQ_WORK.

v2: `depends on SMP` leads to Kconfig warnings when CONFIG_IRQ_WORK is
    selected by something else when !CONFIG_SMP. Use `def_bool y if SMP`
    instead.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Tested-by: Anders Roxell <anders.roxell@linaro.org>
Fixes: 2f34d7337d98 ("workqueue: Fix queue_work_on() with BH workqueues")
Cc: Stephen Rothwell <sfr@canb.auug.org.au>