workqueue: fix typos in comments

tj: dropped iff -> if, iff is if and only if not a typo.  Spotted by
    Randy Dunlap.

Signed-off-by: Shailendra Verma <shailendra.capricorn@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>