carp: carp_master_down_locked() requires net epoch

Reviewed by: kp
Approved by: re (gjb)
Fixes: 1d126e9b9474 carp: Widen epoch coverage
MFC after: 1 day
Differential Revision: https://reviews.freebsd.org/D39113

(cherry picked from commit 49cad3daf2a3abe8270f71ee5b15b8990a19529d)
(cherry picked from commit c15ceae9e7f98f7e74ff7848b3aff7176c7e263a)