net: sockets: Preserve user_data for any net_context send() or recv()

We use ctx->user_data to hold socket flags. As each call to
net_context_recv() and net_context_send() overwrites its previous
value, we explicitly must pass the current ctx value there.

Without this, non-blocking socket was turned into blocking after
e.g. switching from receiving to sending.

Fixes: #6309

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>