fusefs: fix a dead store in fuse_vnop_advlock

kevans actually caught this in the original review and I fixed it, but
then I committed an older copy of the branch. Whoops.

Reported by: kevans
Differential Revision: https://reviews.freebsd.org/D29031

(cherry picked from commit 9c5aac8f2e84ca4bbdf82514302c08c0453ec59b)