perf machine: Use machine__find_kernel_function() instead of open coded version

We have that equivalent, shorter helper, use it.

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: https://lkml.kernel.org/n/tip-1hcgu3k7vxdy4vknqf3kbtzt@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>