cxgbe(4): Always use the per-VI callout to read interface stats.

[np]

Summary

There is no change in the source of the stats (t4_get_port_stats or
t4_get_vi_stats) but the per-port callout is gone.

Sponsored by: Chelsio Communications

[jhb]

sys/dev/cxgbe/t4_main.c

6284

This seems a bit odd as it means you will drop/pickup this lock several times. Maybe make the acquire of this lock in vi_refresh_stats() conditional on IS_VF instead and move the assertion here?

[np]

Acquire reg_lock around read_vf_stat instead of in a loop in the function.