kobject: don't use WARN for registration failures

This WARNING proved to be noisy. The function still returns an error
and callers should handle it. That's how most of kernel code works.
Downgrade the WARNING to pr_err() and leave WARNINGs for kernel bugs.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reported-by:  <syzbot+209c0f67f99fec8eb14b@syzkaller.appspotmail.com>
Reported-by:  <syzbot+7fb6d9525a4528104e05@syzkaller.appspotmail.com>
Reported-by:  <syzbot+2e63711063e2d8f9ea27@syzkaller.appspotmail.com>
Reported-by:  <syzbot+de73361ee4971b6e6f75@syzkaller.appspotmail.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>