mm: memcontrol: Replace disable-IRQ locking with a local_lock

Access to the per-CPU variable memcg_stock is synchronized by disabling
interrupts.

Convert it to a local_lock which allows RT kernels to substitute them with
a real per CPU lock. On non RT kernels this maps to local_irq_save() as
before, but provides also lockdep coverage of the critical region.
No functional change.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>