ZFS: fix assertions with INVARIANTS

AFAICT, this was an oversight from
9e5787d2284e187abb5b654d924394a65772e004 (svn r364746). That revision
inadvertently disabled assertions unconditionally.

Reviewed by: freqlabs
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D28256

(cherry picked from commit 174a7e578a33c01401e33f9bfcc077fc3155251c)