linux: fix reporting NL_RTM_DELLINK to Netlink sockets

The problem is that ifname_bsd_to_linux_name() requires the interface to
exist. But when we are in the context of ifnet_departure_event
EVENTHANDLER(9), it does not. Instead of silently dropping the message,
let's send the FreeBSD name verbatim. At the moment special translation
is done for IFT_LOOPBACK and IFT_ETHER only, and these two kinds of
interfaces usually don't depart. So, this is not a final fix, but
definitely an improvement. While here, simplify the associated code.

Differential Revision: https://reviews.freebsd.org/D51927