fusefs: fix VOP_ADVLOCK with SEEK_END

When the user specifies SEEK_END, unlike SEEK_CUR, VOP_ADVLOCK must
adjust lock offsets itself.

Sort-of related to bug 266886.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37040

(cherry picked from commit f6e5319550f60170840f1a07a9cbdd45b5014a21)