lagg: Handle a port count of zero

The sc_count check in lagg_transmit_ethernet() and
lagg_transmit_infiniband() is racy, as the lagg protocol handlers are
only synchronized by net_epoch. Handle a count of 0 in each protocol
handler where it's needed, namely in the RR and LB handlers.

Reported by: Yuxiang Yang, Yizhou Zhao, Xuewei Feng, Qi Li, and Ke Xu from Tsinghua University using GLM5.1 from Z.ai
Reviewed by: pouria, zlei
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D56942