mm: memcg: shorten preempt-disabled section around event checks

mm: memcg: shorten preempt-disabled section around event checks

Only the ratelimit checks themselves have to run with preemption disabled,
the resulting actions - checking for usage thresholds, updating the soft
limit tree - can and should run with preemption enabled.

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
Reported-by: Yong Zhang <yong.zhang0@gmail.com>
Reported-by: Luis Henriques <henrix@camandro.org>
Tested-by: Luis Henriques <henrix@camandro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>