perf symbols: Protect dso cache tree using dso->lock

The dso cache is accessed during dwarf callchain unwind and it might be
processed concurrently.  Protect it under dso->lock.

Note that it doesn't protect dso_cache__find().  I think it's safe to
access to the cache tree without the lock since we don't delete nodes.

It it missed an existing node due to rotation, it'll find it during
dso_cache__insert() anyway.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1431909055-21442-27-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>