perf top: Access hists->lock only if needed

perf top: Access hists->lock only if needed

The perf_top__record_precise_ip() releases and regrabs the
he->hists->lock because it can sleep if there's an error.  But it should
be done conditionally as it slows down the fast path.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1449802616-16170-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>