net: sockets: Add locking to receive callback

Fix a regression when application is waiting data but does
not notice that because socket layer is not woken up.

This could happen because application was waiting condition
variable but the signal to wake the condvar came before the
wait started. Normally if there is constant flow of incoming
data to the socket, the signal would be given later. But if
the peer is waiting that Zephyr replies, there might be a
timeout at peer.

The solution is to add locking in socket receive callback so
that we only signal the condition variable after we have made
sure that the condition variable is actually waiting the data.

Fixes #34964

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