io_uring/net: add IORING_CQE_F_SOCK_FULL if a send needed to poll arm

If a send/sendmsg/sendzc/sendmsgzc needed to wait for space in the
socket to complete, add IORING_CQE_F_SOCK_FULL to the cqe->flags to tell
the application about it. This meant that the socket was full when the
operation was attempted.

This adds IORING_CQE_F_SOCK_FULL as a new CQE flag. It borrows the value
of IORING_CQE_F_SOCK_NONEMPTY, which is a flag only used on the receive
side. Hence there can be no confusion on which of the two meanings is
included in a given CQE, as the application must know which kind of
operation the completion refers to.

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