posix-timers: Simplify lock/unlock_timer()

posix-timers: Simplify lock/unlock_timer()

Since the integration of sigqueue into the timer struct, lock_timer() is
only used in task context. So taking the lock with irqsave() is not longer
required.

Convert it to use spin_[un]lock_irq().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/all/20250308155623.959825668@linutronix.de