Add support for hardware rate limiting to mlx5en(4).

Add support for hardware rate limiting to mlx5en(4).

The hardware rate limiting feature is enabled by the RATELIMIT kernel
option. Please refer to ifconfig(8) and the txrtlmt option and the
SO_MAX_PACING_RATE set socket option for more information. This
feature is compatible with hardware transmit send offload, TSO.

A set of sysctl(8) knobs under dev.mce.<N>.rate_limit are provided to
setup the ratelimit table and also to fine tune various rate limit
related parameters.

Sponsored by:	Mellanox Technologies