tools lib bpf: Rename bpf_map__get_fd() to bpf_map__fd()

For consistency, leaving "get" for reference counting.

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-msy8sxfz9th6gl2xjeci2btm@git.kernel.org


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