perf disassemble: Move check for kallsyms + !kcore

perf disassemble: Move check for kallsyms + !kcore

We don't need to do all that filename logic to then just have to test
something unrelated and bail out, move it to the start of the function.

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/n/tip-lk1v4srtsktonnyp6t1o0uhx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>