perf evlist: Unmap when all refcounts to fd are gone and events drained

As noticed by receiving a POLLHUP for all its pollfd entries.

That will remove the refcount taken in perf_evlist__mmap_per_evsel(),
and when all events are consumed via perf_evlist__mmap_read() +
perf_evlist__mmap_consume(), the ring buffer will be unmap'ed.

Thanks to Jiri Olsa for pointing out that we must wait till all events
are consumed, not being ok to unmmap just when receiving all the
POLLHUPs.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-t10w1xk4myp7ca7m9fvip6a0@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>