carp: carp_master_down_locked() requires net epoch
Reviewed by: kp
Fixes: 1d126e9b9474 carp: Widen epoch coverage
MFC after: 1 day
Differential Revision: https://reviews.freebsd.org/D39113
(cherry picked from commit 49cad3daf2a3abe8270f71ee5b15b8990a19529d)