lagg: Clean up handling of the rr_limit option.

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