fusefs: flock(2) locks must be implemented in-kernel

If a FUSE file system sets the FUSE_POSIX_LOCKS flag then it can support
fcntl(2)-style locks directly. However, the protocol does not adequately
support flock(2)-style locks until revision 7.17. They must be implemented
locally in-kernel instead. This unfortunately breaks the interoperability
of fcntl(2) and flock(2) locks for file systems that support the former.
C'est la vie.

Prior to this commit flock(2) would get sent to the server as a
fcntl(2)-style lock with the lock owner field set to stack garbage.

Sponsored by: The FreeBSD Foundation