selftests/kernfs: test xattr retrieval

Make sure that listxattr() returns zero and that getxattr() returns
ENODATA when no extended attributs are set. Use /sys/kernel/warn_count
as that always exists and is a read-only file.

Link: https://lore.kernel.org/20250702-hochmoderne-abklatsch-af9c605b57b2@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/filesystems/.gitignore

@@ -3,3 +3,4 @@ dnotify_test
 devpts_pts
 file_stressor
 anon_inode_test
+kernfs_test