ping: fix triptime calculation after r350998

That revision changed the internal clock to the monotonic, but neglected to
change the datagram's timestamp source.

Reported by: Oliver Hartmann, Michael Butler
Reviewed by: Ján Sučan <sucanjan@gmail.com>, allanjude
MFC after: 2 weeks
MFC-With: r350998
Differential Revision: https://reviews.freebsd.org/D21258