ping: Avoid magic numbers
The sizeof(struct ip) is 20.
The sizeof(struct in_addr) is 4.
No functional change intended.
Reviewed by: asomers, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D39125
(cherry picked from commit 491263d7a640a85a6be40fd0c0a9397d072f0847)