MFC r269180:

When interval is set to very small value with limited amount of packets,
ping6(8) would quit before the remote side gets a chance to respond.

Solve this by resetting the itimer when we have reached the maximum packet
number have reached, but let the other handling to continue.

PR:        bin/151023
Submitted by:    tjmao at tjmao.net
MFC r269566:
Fix for deadlock in USB device side mode.