iptuntap: fix multi-queue flag display

iptuntap: fix multi-queue flag display

When creating a tap with multi_queue flag, this flag is not displayed
when dumping:

$ ip tuntap add tap23 mode tap multi_queue
$ ip tuntap
tap23: tap persist0x100

While at it, add a space between known flags and hexdump of unknown
ones.

Fixes: c41e038f48a3 ("iptuntap: allow creation of multi-queue tun/tap device")
Signed-off-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>