bpftool: auto-complete BTF IDs for btf dump

Auto-complete BTF IDs for `btf dump id` sub-command. List of possible BTF
IDs is scavenged from loaded BPF programs that have associated BTFs, as
there is currently no API in libbpf to fetch list of all BTFs in the
system.
Suggested-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/bash-completion/bpftool

@@ -71,6 +71,12 @@ _bpftool_get_prog_tags()
         command sed -n 's/.*"tag": "\(.*\)",$/\1/p' )" -- "$cur" ) )
 }
 
+_bpftool_get_btf_ids()
+{
+    COMPREPLY+=( $( compgen -W "$( bpftool -jp prog 2>&1 | \
+        command sed -n 's/.*"btf_id": \(.*\),\?$/\1/p' )" -- "$cur" ) )
+}
+
 _bpftool_get_obj_map_names()
 {
     local obj
@@ -635,6 +641,9 @@ _bpftool()
                                 map)
                                     _bpftool_get_map_ids
                                     ;;
+                                dump)
+                                    _bpftool_get_btf_ids
+                                    ;;
                             esac
                             return 0
                             ;;