[PR 206219] Kernel panic from lagg_ioctl and lagg_port_ioctl

r287723 removed some cleanup from lagg(4), which leads to panics when
changing configuration. Restore the spirit of the code which was removed.
This issue has been refactored out of existence in -HEAD, so this patch is
directly against stable/10.

PR: 206219
Submitted by: Fred Lewis < flewis @ panasas.com >
Reviewed by: hiren, Daniel O'Connor < darius @ dons.net.au >
Approved by: jhb
Sponsored by: Panasas, Inc.
Differential Revision: https://reviews.freebsd.org/D4929