Do not unlink sanlock lockfile on shutdown
Unlinking a lockfile is racy; another instance may have the same
lockfile open, waiting on the lock.
This is a possible failure mode:
1. Process 1 holds the lock.
2. Process 2 opens the lockfile and tries to acquire the lockfile.
3. Process 1 unlinks the lockfile and releases it.
4. Process 2 acquires the unlinked lockfile.
5. Process 3 creates a new lockfile acquires it.
So we may end with 2 processes, both thinking they are the only instance
running.
This issue may be fixed by checking the lockfile inode before and after
locking to detect deleted lockfile. But there is a simpler solution;
since we have exactly one lockfile, don't delete it.
Signed-off-by: Nir Soffer <nsoffer@redhat.com>