net: skbuff: reject zero MPLS TTL before decrementing

skb_mpls_dec_ttl() decrements the u8 TTL before checking whether it
expired. An incoming TTL of zero wraps to 255, causing the TC MPLS
dec_ttl action to pass an expired packet instead of dropping it.

Reject a zero TTL before the decrement while preserving the existing
behavior for TTL one and valid TTL values.

Fixes: 2a2ea50870ba ("net: sched: add mpls manipulation actions to TC")
Signed-off-by: Avi Weiss <thnkslprpt@gmail.com>
Reviewed-by: Xia Yan <yanxia@cmss.chinamobile.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20261031114135.3784868-2-thnkslprpt@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>