tracing/rb: Init the CPU mask on allocation

tracing/rb: Init the CPU mask on allocation

Before commit b32614c03413 ("tracing/rb: Convert to hotplug state
machine") the allocated cpumask was initialized to the mask of ONLINE or
POSSIBLE CPUs. After the CPU hotplug changes the buffer initialisation
moved to trace_rb_cpu_prepare() but I forgot to initially set the
cpumask to zero. This is done now.

Link: http://lkml.kernel.org/r/20161207133133.hzkcqfllxcdi3joz@linutronix.de
Fixes: b32614c03413 ("tracing/rb: Convert to hotplug state machine")
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>