Adds two FUSE integration tests on the existing dlm cluster harness (the
-dlm mounts route advisory locks to the owner filer):
- TestPosixLockCrossMount: an flock taken on one mount blocks the other,
and is grantable after release — the routed-to-owner path end to end.
- TestPosixLockSurvivesFilerLoss: hold flocks on many files, stop filer1
so keys it owned migrate to filer0; after the ring settles and the
holding mount re-asserts, every lock is still honored. Asserts only the
settled state; the transient migration window is unit-covered.
Locks are taken on read-only fds so the -dlm whole-file write lock (a
different mechanism, held until close) isn't involved. Skipped on
non-Linux: only Linux forwards advisory locks (SETLK) to the FUSE server;
macFUSE handles flock in-kernel per mount.
