netlink: Fix IFF_UP flag handling in RTM_NEWLINK's modify_link handler

IFF_UP could previously only be unset via RTM_NEWLINK. Requests to set
IFF_UP, though they succeeded, did not actually set the flag.

Reviewed by: obiwac, kp, mckusick (mentor)
Approved by: obiwac, kp, mckusick (mentor)
Sponsored by: Google LLC (GSoC)
Differential Revision: https://reviews.freebsd.org/D51871