icmp: clear offset and flags when reflecting a packet

When reflecting a packet, use an offset of 0 and clear all three bits,
in particular the DF bit.

PR: 288558
Reviewed by: markj, zlei
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D51991

(cherry picked from commit b9a2d84b1bf7f9cf556e2f0b68023d5af8362797)