if_media.c SIOCGMEDIAX handler: improve loop

Stop advancing counter past the current iteration number at the start
of iteration. This removes the need of subtracting one when
calculating index for copyout, and arguably fixes off-by-one reporting
of copied out elements when copyout failed.

Reviewed by: hselasky
Sponsored by: Mellanox Technologies / NVidia Networking
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27073