bpftool: Improve handling of ENOSPC on reuseport_array map dumps

avoids outputting a series of
	value:
	No space left on device

The value itself is not wrong but bpf_fd_reuseport_array_lookup_elem() can
only return it if the map was created with value_size = 8. There's nothing
bpftool can do about it. Instead of repeating this error for every key in
the map, print an explanatory warning and a specialized error.

example before:
key: 00 00 00 00
value:
No space left on device
key: 01 00 00 00
value:
No space left on device
key: 02 00 00 00
value:
No space left on device
Found 0 elements

example after:
Warning: cannot read values from reuseport_sockarray map with value_size != 8
key: 00 00 00 00  value: <cannot read>
key: 01 00 00 00  value: <cannot read>
key: 02 00 00 00  value: <cannot read>
Found 0 elements
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/bpf/bpftool/map.c

@@ -719,6 +719,9 @@ static int dump_map_elem(int fd, void *key, void *value,
 
 		if (errno == ENOENT)
 			msg = "<no entry>";
+		else if (lookup_errno == ENOSPC &&
+			 map_info->type == BPF_MAP_TYPE_REUSEPORT_SOCKARRAY)
+			msg = "<cannot read>";
 
 		print_entry_error(map_info, key,
 				  msg ? : strerror(lookup_errno));
@@ -775,6 +778,10 @@ static int do_dump(int argc, char **argv)
 			}
 		}
 
+	if (info.type == BPF_MAP_TYPE_REUSEPORT_SOCKARRAY &&
+	    info.value_size != 8)
+		p_info("Warning: cannot read values from %s map with value_size != 8",
+		       map_type_name[info.type]);
 	while (true) {
 		err = bpf_map_get_next_key(fd, prev_key, key);
 		if (err) {