inet6: Fix LINT build

mli_delete_locked() is the only function that takes a const ifnet.
Since it's a static function there's no advantage to keeping it const.
Since if_t is not a const struct (currently) the compiler throws an
error passing the ifp around to ifnet functions.

Fixes: eb1da3e5258238e1c0555c6a006a341df0821d8e
Sponsored by: Juniper Networks, Inc.