fusefs: correctly set lock owner during FUSE_SETLK

During FUSE_SETLK, the owner field should uniquely identify the calling
process. The fusefs module now sets it to the process's pid.
Previously, it expected the calling process to set it directly, which
was wrong.

libfuse also apparently expects the owner field to be set during
FUSE_GETLK, though I'm not sure why.

PR: 256005
Reported by: Agata <chogata@moosefs.pro>
MFC after: 2 weeks
Reviewed by: pfg
Differential Revision: https://reviews.freebsd.org/D30622