perf unwind: Unwind with libdw doesn't take symfs into account

Path passed to libdw for unwinding doesn't include symfs path
if specified, so unwinding fails because ELF file is not found.

Similar to unwinding with libunwind, pass symsrc_filename instead
of long_name. If there is no symsrc_filename, fallback to long_name.

Signed-off-by: Martin Vuille <jpmv27@aim.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20180211212420.18388-1-jpmv27@aim.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>