tcp: Use skb__nullable in trace_tcp_send_reset

tcp: Use skb__nullable in trace_tcp_send_reset

Replace skb with skb__nullable as the argument name. The suffix tells
bpf verifier through btf that the arg could be NULL and should be
checked in tp_btf prog.

For now, this is the only nullable argument in tcp tracepoints.

Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20240911033719.91468-4-lulie@linux.alibaba.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>