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
MFC after: 13 days
MFC with: 929acdb19acb67cc0e6ee5439df98e28a84d4772
Differential Revision: https://reviews.freebsd.org/D29031