perf tools: Fix perf record as non root with kptr_restrict == 1

Currently perf record always errors out when you run it as non-root with
kptr_restrict == 1, which is often the default.

Make it only warn instead and fix the kernel resolve code to not
segfault later. Profiling works still fine, except kernel symbols are
not resolved.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1411594794-7229-1-git-send-email-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>