Return Truncate error instead of looping forever on udp send.

[Easyoakland]

Issue

If the udpsocket send methods are given a buffer larger than the udp payload buffer the methods loop await forever trying to send and getting BufferFull.

Fix

Check and early-return if udp::send methods receive a buffer too large to ever be sent.

Details

smoltcp used to distinguish these but stopped here. I'm not sure why. You can still the the remnant in the now invalid comment for smoltcp::socket::udp::Socket::send mentioning Truncate. I don't know if it would be preferred to update smoltcp or leave the fix here in embassy-net.

[Dirbaio]

thank you!

[Easyoakland]

Oh, I forgot to update the comments to match the name change. Should I make a new Pr for that?

[Dirbaio]

oh, yes please! btw could you also remove the Poll::Ready mention? it's already implied that when an async function returns it has returned Poll::Ready.