net: sockets: Store socket private data into its own variable

Do not try to re-use net_context.user_data field as in many places
(like in accept) it is expected to contain pointer to net_context.
Storing the socket flags will corrupt the value. To simplify and
make things less error prone, use socket specific field in net_context
to store the socket flags.

Fixes #19191

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>