dhcpcd: add diagnostics to valid_udp_packet

A UDP packet may be invalid for a number of reasons. When a packet
is found to be invalid, log the specific reason that we rejected it.

BUG=chromium:342102
TEST=network_DhcpRenew (doesn't test new functionality)

Change-Id: I8f4633cd3974fd10c2bada90842e9902dea0ea1c
Reviewed-on: https://chromium-review.googlesource.com/185511
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Tested-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: mukesh agrawal <quiche@chromium.org>
1 file changed