fixup! fixup! KTOR-1159 Add support for UDP sockets on native

Workaround for Kotlin/kotlinx.coroutines#2263

Author: Thomas-Vos

ktor-network/posix/src/io/ktor/network/sockets/DatagramSendChannel.kt

@@ -25,9 +25,9 @@ internal class DatagramSendChannel(
 
     @ExperimentalCoroutinesApi
     override val isFull: Boolean
-        get() = if (isClosedForSend) false else lock.isLocked
+        get() = if (isClosedForSend) false else lock.availablePermits == 0
 
-    private val lock = Mutex()
+    private val lock = Semaphore(1)
 
     override fun close(cause: Throwable?): Boolean {
 //        if (socket.isClosed) {
@@ -44,7 +44,7 @@ internal class DatagramSendChannel(
     }
 
     override suspend fun send(element: Datagram) {
-        lock.withLock {
+        lock.withPermit {
             sendImpl(element)
         }
     }