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)