timers: Split [try_to_]del_timer[_sync]() to prepare for shutdown mode

Tearing down timers which have circular dependencies to other
functionality, e.g. workqueues, where the timer can schedule work and work
can arm timers, is not trivial.

In those cases it is desired to shutdown the timer in a way which prevents
rearming of the timer. The mechanism to do so is to set timer->function to
NULL and use this as an indicator for the timer arming functions to ignore
the (re)arm request.

Split the inner workings of try_do_del_timer_sync(), del_timer_sync() and
del_timer() into helper functions to prepare for implementing the shutdown
functionality.

No functional change.

Co-developed-by: Steven Rostedt <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Reviewed-by: Jacob Keller <[email protected]>
Reviewed-by: Anna-Maria Behnsen <[email protected]>
Link: https://lore.kernel.org/all/[email protected]
Link: https://lore.kernel.org/all/[email protected]
Link: https://lore.kernel.org/r/[email protected]

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