tools/bpftool: Add bpftool support for bpf map element iterator

The optional parameter "map MAP" can be added to "bpftool iter"
command to create a bpf iterator for map elements. For example,
  bpftool iter pin ./prog.o /sys/fs/bpf/p1 map id 333

For map element bpf iterator "map MAP" parameter is required.
Otherwise, bpf link creation will return an error.

Quentin Monnet kindly provided bash-completion implementation
for new "map MAP" option.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200723184119.590799-1-yhs@fb.com

tools/bpf/bpftool/bash-completion/bpftool

@@ -615,7 +615,23 @@ _bpftool()
         iter)
             case $command in
                 pin)
-                    _filedir
+                    case $prev in
+                        $command)
+                            _filedir
+                            ;;
+                        id)
+                            _bpftool_get_map_ids
+                            ;;
+                        name)
+                            _bpftool_get_map_names
+                            ;;
+                        pinned)
+                            _filedir
+                            ;;
+                        *)
+                            _bpftool_one_of_list $MAP_TYPE
+                            ;;
+                    esac
                     return 0
                     ;;
                 *)