mlx4en: Use the common RSS key for programming and queries

[kbowling]

Summary

Replace the private fixed key with rss_getkey().  Copy the common byte
stream directly into the RSS queue-pair context and use the same helper
for SIOCGIFRSSKEY, so hardware programming and hn's query agree.

Change the helper to fill caller-owned storage instead of returning a
pointer to a static key.  Check the fixed hardware and ioctl buffer sizes
at compile time.  Leave RSS masks and queue steering unchanged.

MFC after:      2 weeks
Sponsored by:   BBOX.io

[kib]

Nvidia does not maintain the mlx4 driver, and I cannot even test it. Change it on your own discretion.

sys/dev/mlx4/mlx4_en/mlx4_en_netdev.c

2095

Why CTASSERT() and not KASSERT()?

[kib]

sys/dev/mlx4/mlx4_en/mlx4_en_netdev.c

2095

I mean _Static_assert() of course.