locking/rtmutex: Clean up signal handling in __rt_mutex_slowlock()

The signal handling in __rt_mutex_slowlock() is open coded.

Use signal_pending_state() instead.

Aside of the cleanup this also prepares for the RT lock substituions which
require support for TASK_KILLABLE.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210326153944.533811987@linutronix.de