kernel: shim-udp4: fix interactions with vertical flow control (-EAGAIN)
kernel: shim-udp4: fix interactions with vertical flow control (-EAGAIN)

This also simplifies the transmission path, removing the need to use a
workqueue and all the related code.

autrimpo/rlite,vmaffione/rlite,vmaffione/rlite,autrimpo/rlite,vmaffione/rlite,vmaffione/rlite,autrimpo/rlite,autrimpo/rlite