tools lib bpf: Use IS_ERR() reporting macros with bpf_map__get_def()

tools lib bpf: Use IS_ERR() reporting macros with bpf_map__get_def()

And for consistency, rename it to bpf_map__def(), leaving "get" for
reference counting.

Also make it return a const pointer, as suggested by Wang.

Acked-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/n/tip-mer00xqkiho0ymg66b5i9luw@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>