Add an IOCTL rr_limit to let users fine tuning the number of packets to be
sent using roundrobin protocol and set a better granularity and distribution
among the interfaces. Tuning the number of packages sent by interface can
increase throughput and reduce unordered packets as well as reduce SACK.

Example of usage:

1. ifconfig bge0 up
2. ifconfig bge1 up
3. ifconfig lagg0 create
4. ifconfig lagg0 laggproto roundrobin laggport bge0 laggport bge1 \

192.168.1.1 netmask 255.255.255.0

ifconfig lagg0 rr_limit 500

Reviewed by: thompsa, glebius, adrian (old patch)
Approved by: bapt (mentor)
Relnotes: Yes
Differential Revision: https://reviews.freebsd.org/D540