9pfs-local: simplify/optimize local_mapped_attr_path()

Omit one unnecessary memory allocation for components
of the path and create the resulting path directly given
lengths of the components.

Do not use basename(3) because there are 2 versions of
this function which differs when argument ends with
slash character, use strrchr() instead so we have
consistent result.  This also makes sure the function
will do the right thing in corner cases (eg, empty
pathname is given), when basename(3) return entirely
another string.
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

hw/9pfs/virtio-9p-local.c

@@ -45,19 +45,17 @@
 
 static char *local_mapped_attr_path(FsContext *ctx, const char *path)
 {
-    char *dir_name;
-    char *tmp_path = g_strdup(path);
-    char *base_name = basename(tmp_path);
-    char *buffer;
-
-    /* NULL terminate the directory */
-    dir_name = tmp_path;
-    *(base_name - 1) = '\0';
-
-    buffer = g_strdup_printf("%s/%s/%s/%s",
-             ctx->fs_root, dir_name, VIRTFS_META_DIR, base_name);
-    g_free(tmp_path);
-    return buffer;
+    int dirlen;
+    const char *name = strrchr(path, '/');
+    if (name) {
+        dirlen = name - path;
+        ++name;
+    } else {
+        name = path;
+        dirlen = 0;
+    }
+    return g_strdup_printf("%s/%.*s/%s/%s", ctx->fs_root,
+                           dirlen, path, VIRTFS_META_DIR, name);
 }
 
 static FILE *local_fopen(const char *path, const char *mode)