sysroot: Make devpath more clear

Coverity points out that we have a memory leak from g_strdup(devpath).
This is a mistake from coverity. However, we use `g_path_get_dirname()`
which makes the process clearer since it does not modify the string in
place.

src/libostree/ostree-sysroot.c

@@ -2243,7 +2243,7 @@ ostree_sysroot_deployment_unlock (OstreeSysroot *self, OstreeDeployment *deploym
                         deployment, _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_FLAG_DEVELOPMENT)
                   : _ostree_sysroot_get_runstate_path (
                         deployment, _OSTREE_SYSROOT_DEPLOYMENT_RUNSTATE_FLAG_TRANSIENT);
-        g_autofree char *devpath_parent = dirname (g_strdup (devpath));
+        g_autofree char *devpath_parent = g_path_get_dirname (devpath);
 
         if (!glnx_shutil_mkdir_p_at (AT_FDCWD, devpath_parent, 0755, cancellable, error))
           return FALSE;