perf trace: Check control+C more often

We were checking for it only after processing all events in the buffer,
delaying processing the termination request for long periods.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-9jdbu937curvb35cfzbyss4g@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>