net: hsr: avoid possible NULL deref in skb_clone()

syzbot got a crash [1] in skb_clone(), caused by a bug
in hsr_get_untagged_frame().

When/if create_stripped_skb_hsr() returns NULL, we must
not attempt to call skb_clone().

While we are at it, replace a WARN_ONCE() by netdev_warn_once().

[1]
general protection fault, probably for non-canonical address 0xdffffc000000000f: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000078-0x000000000000007f]
CPU: 1 PID: 754 Comm: syz-executor.0 Not tainted 6.0.0-syzkaller-02734-g0326074f #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/22/2022
RIP: 0010:skb_clone+0x108/0x3c0 net/core/skbuff.c:1641
Code: 93 02 00 00 49 83 7c 24 28 00 0f 85 e9 00 00 00 e8 5d 4a 29 fa 4c 8d 75 7e 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <0f> b6 04 02 4c 89 f2 83 e2 07 38 d0 7f 08 84 c0 0f 85 9e 01 00 00
RSP: 0018:ffffc90003ccf4e0 EFLAGS: 00010207

RAX: dffffc0000000000 RBX: ffffc90003ccf5f8 RCX: ffffc9000c24b000
...