net/x25: Fix skb leak in x25_lapb_receive_frame()

x25_lapb_receive_frame() using skb_copy() to get a private copy of
skb, the new skb should be freed in the undersized/fragmented skb
error handling path. Otherwise there is a memory leak.

Fixes: cb101ed2

 ("x25: Handle undersized/fragmented skbs")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20221114110519.514538-1-weiyongjun@huaweicloud.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>