perf/core: Fix perf_event_enable_on_exec() timekeeping (again)

perf/core: Fix perf_event_enable_on_exec() timekeeping (again)

Where commit:

  bed0401b2ac7 ("perf: Fix scaling vs.  perf_event_enable_on_exec()")

disabled the ctx-time a-priory, such that all events get enabled and
scheduled at the time point in time, there is one hole in that patch,
when no events do get enabled nothing re-enables the ctx-time.

Reported-by: Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Reported-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vince Weaver <vincent.weaver@maine.edu>
Fixes: bed0401b2ac7 ("perf: Fix scaling vs.  perf_event_enable_on_exec()")
Signed-off-by: Ingo Molnar <mingo@kernel.org>