workqueue: avoid using deprecated functions

The network merge brought in a few users of functions that got
deprecated by the workqueue cleanups: the 'system_nrt_wq' is now the
same as the regular system_wq, since all workqueues are now non-
reentrant.

Similarly, remove one use of flush_work_sync() - the regular
flush_work() has become synchronous, and the "_sync()" version is thus
deprecated as being superfluous.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>