ping: Fix the display of Flags/Fragment Offset

In the IP header, Flags + Fragment Offset is a 16-bit field.

Use ntohs() instead of ntohl(), otherwise the Flags/Fragment Offset
values may not display correctly.

Before (DF set)

Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 0001   0 0000  40  01 b6a4 192.0.2.1  192.0.2.2

After (DF set)

Vr HL TOS  Len   ID Flg  off TTL Pro  cks      Src      Dst
 4  5  00 0054 0001   2 0000  40  01 b6a4 192.0.2.1  192.0.2.2

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38479