Commit 8a63d29c removed `devif_iob_send` from `udp_sendto_buffered`
workflow, `devif_iob_send` drops too big packet. Now we still need a
place to check the packet length, otherwise a packet larger than MTU
may be sent to the net driver.
In case of similar problem happens somewhere else, this commit also
adds a check in `netdev_upperhalf`, and count these cases into
`NETDEV_TXERRORS`.
Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>
22b6076f26