net: sockets: Keep lock when notifying condvar

Releasing the lock before notifying condvar led to a race condition
between a thread calling k_condvar_wait to wait for a condition variable
and another thread signalling for this same condition variable. This
resulted in the waiting thread to stay pending and the handle to it
getting removed from the notifyq, meaning it couldn't get woken up
again.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>