fusefs: After successful F_GETLK, l_whence should be SEEK_SET

PR: 266886
Reported by: John Millikin <jmillikin@gmail.com>
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D37014

(cherry picked from commit 3c3b906b54236841d813fd9a01b1e090f39558ea)

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)

Approved by: kib (re)