workqueue: Introduce from_work() helper for cleaner callback declarations

To streamline the transition from tasklets to worqueues, a new helper
function, from_work(), is introduced. This helper, inspired by existing
from_() patterns, utilizes container_of() and eliminates the redundancy
of declaring variable types, leading to more concise and readable code.

The modified code snippet demonstrates the enhanced clarity achieved
with from_wq():

  void callback(struct work_struct *w)
   {
     - struct some_data_structure *local = container_of(w,
						       struct some_data_structure,
						       work);
     + struct some_data_structure *local = from_work(local, w, work);

This change aims to facilitate a smoother transition and uphold code
quality standards.

Based on:
  git://git.kernel.org/pub/scm/linux/kernel/git/tj/wq.git disable_work-v3

Signed-off-by: Allen Pais <allen.lkml@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: allenpais

include/linux/workqueue.h

@@ -522,6 +522,9 @@ alloc_workqueue(const char *fmt, unsigned int flags, int max_active, ...);
 #define create_singlethread_workqueue(name)				\
 	alloc_ordered_workqueue("%s", __WQ_LEGACY | WQ_MEM_RECLAIM, name)
 
+#define from_work(var, callback_work, work_fieldname)	\
+	container_of(callback_work, typeof(*var), work_fieldname)
+
 extern void destroy_workqueue(struct workqueue_struct *wq);
 
 struct workqueue_attrs *alloc_workqueue_attrs(void);