netstat: report undelivered multi and broadcast UDP packets correctly

When reporting undelivered broadcast/multicast datagrams, don't just
report the undelivered broadcast ones. Actually report the sum of both.
Alternatively, one could also report broadcast and multicast datagrams
separately, but that would change the output format of netstat.
While there, use the correct type for casting a variable printed
with %ju.

Reviewed by: cc
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D51881