net: socket service resets its restart flag

The socket service restarts whenever a new socket is registered.
This is triggered via zvfs_eventfd_write(ctx.events[0].fd, 1),
but the restart flag (ctx.events[0].revents) is not properly cleared,
causing unintended repeated restarts and skipped socket processing.

Detecting socket restart is done after the socket fd has processed its
data, so the socket doesn't lose its data. Ensure the socket service
clears its restart flag (ctx.events[0].revents = 0) immediately after
detecting a restart event, preventing unnecessary restarts.

Fixes #81813

Signed-off-by: Dominic Moffat <dom@illysky.com>