VPP-621: ping: ICMP echo data size must be bounded by VLIB_BUFFER_DATA_SIZE minus...

VPP-621: ping: ICMP echo data size must be bounded by VLIB_BUFFER_DATA_SIZE minus headers.

Before the commit 878c6098 the VLIB_BUFFER_DATA_SIZE was different depending
on whether building "vpp" or "vpp_lite", resulting in an overrun in vpp_lite build.
Avoid the hardcoded value and make the upper bound for ICMP echo data size
dependent on the buffer size.

Change-Id: Id6c4d7fc73766a95af2610eb237881b5fe9ce9aa
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>