lagg: Clean up handling of the rr_limit option.
- Don't allow an unprivileged user to set the stride. [1]
- Only set the stride under the softc lock.
- Rename the internal fields to accurately reflect their use. Keep ro_bkt to avoid changing the user API.
- Simplify the implementation. The port index is just sc_seq / stride.
- Document rr_limit in ifconfig.8.
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com> [1]
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D22857