kobject: don't use WARN for registration failures

[ Upstream commit 3e14c6ab ]

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>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>