Load IO error count for posix atomically (#4448)

* Load error count atomically in order to check for maximum allowed number of IO errors.

* Remove unused (previously atomic) network IO error count

cmd/storage-rpc-client.go

@@ -28,7 +28,6 @@ import (
 )
 
 type networkStorage struct {
-	networkIOErrCount int32 // ref: https://golang.org/pkg/sync/atomic/#pkg-note-BUG
 	rpcClient         *AuthRPCClient
 }