selftests: drv-net: use env.rpath in the HDS test

Commit 29b036be1b ("selftests: drv-net: test XDP, HDS auto and
the ioctl path") added a new test case in the net tree, now that
this code has made its way to net-next convert it to use the env.rpath()
helper instead of manually computing the relative path.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250228212956.25399-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/hds.py

@@ -20,8 +20,7 @@ def _get_hds_mode(cfg, netnl) -> str:
 
 
 def _xdp_onoff(cfg):
-    test_dir = os.path.dirname(os.path.realpath(__file__))
-    prog = test_dir + "/../../net/lib/xdp_dummy.bpf.o"
+    prog = cfg.rpath("../../net/lib/xdp_dummy.bpf.o")
     ip("link set dev %s xdp obj %s sec xdp" %
        (cfg.ifname, prog))
     ip("link set dev %s xdp off" % cfg.ifname)