fusefs: during F_GETLK, don't change l_pid if no lock is found

PR: 266885
Submitted by: John Millikin <jmillikin@gmail.com>
Sponsored by: Axcient
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D36905

(cherry picked from commit 46fcf947c6c8db1e1ceb3cbd75b69d1d1e494929)