Test cases:
0. Basic non-breaking read/write leases.
1. Multiple read leases and OFDs with no lease for the same file.
2. Breaking leases.
3. Multiple fds (dup + inherited) for single lease (mutual OFD).
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>