locks: Remove duplicated locks

CRIU creates dictinct lock record for each file descriptor on the same
OFD. The patch removes this duplicates. To do so, it adds new field into
struct file_lock, which stores pid of fd, on which lock was found.
'owner pid' is not actually helpful, because the original fd, on which
lock have been set, can be already closed.

Also it purges crutches doing the same stuff but only for file leases.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>