perf probe: Use return of map__get() to make code more compact

The __get() idiom returns a reference count for the object passed, i.e.
all functions of this type return the object passed, so take advantage
of that to make the code more compact.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-ds6vdm7clh070512rpydidsc@git.kernel.org


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