Make NativeLibraryLoader check java.library.path first

Motivation:

On restricted systems (e.g. grsecurity), it might not be possible to write a .so on disk and load it afterwards. On those system Netty should check java.library.path for libraries to load.

Modifications:

Changed NativeLibraryLoader.java to first try to load libs from java.library.path before exporting the .so to disk.

Result:

Libraries load fine on restricted systems.

common/src/main/java/io/netty/util/internal/NativeLibraryLoader.java

@@ -113,6 +113,17 @@ public final class NativeLibraryLoader {
         // Adjust expected name to support shading of native libraries.
         String name = calculatePackagePrefix().replace('.', '_') + originalName;
 
+        try {
+            // first try to load from java.library.path
+            loadLibrary(loader, name, false);
+            logger.debug("{} was loaded from java.libary.path", name);
+            return;
+        } catch (Throwable ex) {
+            logger.debug(
+                    "{} cannot be loaded from java.libary.path, "
+                    + "now trying export to -Dio.netty.native.workdir: {}", name, WORKDIR, ex);
+        }
+
         String libname = System.mapLibraryName(name);
         String path = NATIVE_RESOURCE_HOME + libname;
 
@@ -125,12 +136,6 @@ public final class NativeLibraryLoader {
             }
         }
 
-        if (url == null) {
-            // Fall back to normal loading of JNI stuff
-            loadLibrary(loader, name, false);
-            return;
-        }
-
         int index = libname.lastIndexOf('.');
         String prefix = libname.substring(0, index);
         String suffix = libname.substring(index, libname.length());