perf symbols: Check '/tmp/perf-' symbol file ownership

The external symbol files are generated by JIT compilers, for example, but we
need to make sure they're ours before injecting them to 'perf report'.

Requested-by: Ingo Molnar <mingo@elte.hu>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1312919658-17158-1-git-send-email-penberg@kernel.org
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>