ptp: vclock: Switch from RCU to SRCU

ptp: vclock: Switch from RCU to SRCU

The usage of PTP vClocks leads immediately to the following issues with
ptp4l with LOCKDEP and DEBUG_ATOMIC_SLEEP enabled: "BUG: sleeping function
called from invalid context".

ptp_convert_timestamp() acquires a mutex_t within a RCU read section.  This
is illegal, because acquiring a mutex_t can result in voluntary scheduling
request which is not allowed within a RCU read section.

Replace the RCU usage with SRCU where sleeping is allowed.

Reported-by: Florian Zeitz <florian.zeitz@schettke.com>
Closes: https://lore.kernel.org/all/00a8cce8-410e-4038-98af-49be6d93d7bd@schettke.com/
Fixes: 67d93ffc0f3c ("ptp: vclock: use mutex to fix "sleep on atomic" bug")
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260529-vclock_rcu-v2-1-02a5531fab92@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>