infiniband: Appease Coverty

Coverity claims the call to rdma_gid2ip in cma_igmp_send overwrites addr.
Use a consistent definition of sockaddr to prevent detections and code
changes in the future.

Submitted by: bret_ketchum@dell.com
Reported by: Coverity
Reviewed by: hselasky, kib
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D26229