io_uring/net: send retry for zerocopy

io_uring handles short sends/recvs for stream sockets when MSG_WAITALL
is set, however new zerocopy send is inconsistent in this regard, which
might be confusing. Handle short sends.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b876a4838597d9bba4f3215db60d72c33c448ad0.1659622472.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>