lib/percpu_ida.c: use _irqsave() instead of local_irq_save() + spin_lock

lib/percpu_ida.c: use _irqsave() instead of local_irq_save() + spin_lock

percpu_ida() decouples disabling interrupts from the locking operations.
This breaks some assumptions if the locking operations are replaced like
they are under -RT.

The same locking can be achieved by avoiding local_irq_save() and using
spin_lock_irqsave() instead.  percpu_ida_alloc() gains one more preemption
point because after unlocking the fastpath and before the pool lock is
acquired, the interrupts are briefly enabled.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Nicholas Bellinger <[email protected]>
Cc: Shaohua Li <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Jens Axboe <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>