icmp: bug fix of buffer->error

icmp: bug fix of buffer->error

Recent changes in icmp4/6 choose to free the original
buffer, and make a copy for sending icmp reply back.
However, buffer->error will be ignored when the buffer
is freed unconditionally.

A quick fix can be moving the counter increment code to icmp,
but I prefert to enqueue all buffers to 'error-drop' so that
they can be handled in a batch

rebase, using vlib_buffer_enqueue_to_single_next

Change-Id: I9f3028b55f1d5f634763e2410cd91e17f368195e
Signed-off-by: Kingwel Xie <kingwel.xie@ericsson.com>