lo: Use new KPI to assign the first loop interface

While here remove a pointless static local variable lo_cloner.

No functional change intended.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D45728

(cherry picked from commit 2cb7605a2415a1c0dd35374a88f815ef00e31de9)