Made sure upload does not fail on filesystems that do not support locking.

Closes #2609414

src/Controller/UploadController.php

@@ -177,7 +177,7 @@ class UploadController extends ControllerBase {
     }
 
     // Acquire an exclusive lock.
-    if (!flock($fp, LOCK_EX)) {
+    if (stream_supports_lock($fp) && !flock($fp, LOCK_EX)) {
       fclose($fp);
       throw new LockAcquiringException("Couldn't acquire an exclusive lock for ". FileWidgetFormAlter::getUploadUri($upload) . ".");
     }
@@ -194,7 +194,9 @@ class UploadController extends ControllerBase {
     }
     catch (\Exception $e) {
       $transaction->rollback();
-      flock($fp, LOCK_UN);
+      if (stream_supports_lock($fp)) {
+        flock($fp, LOCK_UN);
+      }
       fclose($fp);
       throw new FileWriteException('Failed to write chunk.');
     }
@@ -204,7 +206,9 @@ class UploadController extends ControllerBase {
 
     // Flush the output then unlock and close the file.
     fflush($fp);
-    flock($fp, LOCK_UN);
+    if (stream_supports_lock($fp)) {
+      flock($fp, LOCK_UN);
+    }
     fclose($fp);
 
     // Return the updated number of uploaded chunks.