workqueue: use new hashtable implementation

Switch workqueues to use the new hashtable implementation. This reduces the
amount of generic unrelated code in the workqueues.

This patch depends on ("hashtable: introduce a small and naive
hashtable") which was merged in v3.6.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>

https://github.com/neobuddy89/hammerhead/commit/ca5fcfbb29a05842ad19cee948703587e9a96762#diff-35b5a45e4113ec78c62d17c3ad246ed2R3818