rhashtable: introduce rhashtable_wakeup_worker helper function

Introduce rhashtable_wakeup_worker() helper function to reduce
duplicated code where to wake up worker.

By the way, as long as the both "future_tbl" and "tbl" bucket table
pointers point to the same bucket array, we should try to wake up
the resizing worker thread, otherwise, it indicates the work of
resizing hash table is not finished yet. However, currently we will
wake up the worker thread only when the two pointers point to
different bucket array. Obviously this is wrong. So, the issue is
also fixed as well in the patch.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Cc: Thomas Graf <tgraf@suug.ch>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>