rafs: optimize Node::name() to reduce image build time

According to perf flamegraph, Node::name() costs too much time
when generating nydus images from tar files. So optimize it.

Signed-off-by: Jiang Liu <gerry@linux.alibaba.com>

rafs/src/builder/core/node.rs

@@ -711,7 +711,10 @@ impl Node {
 
     /// Get filename of the inode.
     pub fn name(&self) -> &OsStr {
-        if self.path() == &self.info.source {
+        let targets = self.target_vec();
+        if let Some(name) = targets.last() {
+            name
+        } else if self.path() == &self.info.source {
             OsStr::from_bytes(ROOT_PATH_NAME)
         } else {
             // Safe to unwrap because `path` is returned from `path()` which is canonicalized