Fix ip_output() on point-to-point links

In r304435, ip_output() was changed to use the result of the route
lookup to decide whether the outgoing packet was a broadcast or
not. This introduced a regression on interfaces where
IFF_BROADCAST was not set (e.g. point-to-point links), as the
algorithm could incorrectly treat the destination address as a
broadcast address, and ip_output() would subsequently drop the
packet as broadcasting on a non-IFF_BROADCAST interface is not
allowed.

Differential Revision: https://reviews.freebsd.org/D8303
Reviewed by: jtl
Reported by: ambrisko
MFC after: 2 weeks
X-MFC-With: r304435
Sponsored by: Dell EMC Isilon