Simplify ReceiveMessageFromAsync signature

Do the slicing at the call sites to avoid passing in a separate offset and count.

Author: stephentoub

src/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncContext.Unix.cs

@@ -1282,12 +1282,10 @@ public SocketError ReceiveMessageFrom(
             return operation.ErrorCode;
         }
 
-        public SocketError ReceiveMessageFromAsync(Memory<byte> buffer, IList<ArraySegment<byte>> buffers, int offset, int count, SocketFlags flags, byte[] socketAddress, ref int socketAddressLen, bool isIPv4, bool isIPv6, out int bytesReceived, out SocketFlags receivedFlags, out IPPacketInformation ipPacketInformation, Action<int, byte[], int, SocketFlags, IPPacketInformation, SocketError> callback)
+        public SocketError ReceiveMessageFromAsync(Memory<byte> buffer, IList<ArraySegment<byte>> buffers, SocketFlags flags, byte[] socketAddress, ref int socketAddressLen, bool isIPv4, bool isIPv6, out int bytesReceived, out SocketFlags receivedFlags, out IPPacketInformation ipPacketInformation, Action<int, byte[], int, SocketFlags, IPPacketInformation, SocketError> callback)
         {
             SetNonBlocking();
 
-            buffer = buffer.Slice(offset, count);
-
             SocketError errorCode;
             int observedSequenceNumber;
             if (_receiveQueue.IsReady(this, out observedSequenceNumber) &&

src/System.Net.Sockets/src/System/Net/Sockets/SocketAsyncEventArgs.Unix.cs

@@ -211,7 +211,7 @@ internal unsafe SocketError DoOperationReceiveMessageFrom(Socket socket, SafeClo
             int bytesReceived;
             SocketFlags receivedFlags;
             IPPacketInformation ipPacketInformation;
-            SocketError socketError = handle.AsyncContext.ReceiveMessageFromAsync(_buffer, _bufferListInternal, _offset, _count, _socketFlags, _socketAddress.Buffer, ref socketAddressSize, isIPv4, isIPv6, out bytesReceived, out receivedFlags, out ipPacketInformation, ReceiveMessageFromCompletionCallback);
+            SocketError socketError = handle.AsyncContext.ReceiveMessageFromAsync(_buffer.Slice(_offset, _count), _bufferListInternal, _socketFlags, _socketAddress.Buffer, ref socketAddressSize, isIPv4, isIPv6, out bytesReceived, out receivedFlags, out ipPacketInformation, ReceiveMessageFromCompletionCallback);
             if (socketError != SocketError.IOPending)
             {
                 CompleteReceiveMessageFromOperation(bytesReceived, _socketAddress.Buffer, socketAddressSize, receivedFlags, ipPacketInformation, socketError);

src/System.Net.Sockets/src/System/Net/Sockets/SocketPal.Unix.cs

@@ -1676,7 +1676,7 @@ public static SocketError ReceiveMessageFromAsync(Socket socket, SafeCloseSocket
             int bytesReceived;
             SocketFlags receivedFlags;
             IPPacketInformation ipPacketInformation;
-            SocketError socketError = handle.AsyncContext.ReceiveMessageFromAsync(buffer, null, offset, count, socketFlags, socketAddress.Buffer, ref socketAddressSize, isIPv4, isIPv6, out bytesReceived, out receivedFlags, out ipPacketInformation, asyncResult.CompletionCallback);
+            SocketError socketError = handle.AsyncContext.ReceiveMessageFromAsync(new Memory<byte>(buffer, offset, count), null, socketFlags, socketAddress.Buffer, ref socketAddressSize, isIPv4, isIPv6, out bytesReceived, out receivedFlags, out ipPacketInformation, asyncResult.CompletionCallback);
             if (socketError == SocketError.Success)
             {
                 asyncResult.CompletionCallback(bytesReceived, socketAddress.Buffer, socketAddressSize, receivedFlags, ipPacketInformation, SocketError.Success);