epoch: replace hand coded assertion

The assertion is equivalent to kstack_contains() so use that rather
than spelling it out.

Suggested by: jhb
Reviewed by: jhb
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D38107

(cherry picked from commit fa1d803c0f652d72840a3c59139baf9d30792860)