fcntl(F_SETSIG) fix

fcntl(F_SETSIG) no longer works on leases because
lease_release_private_callback() gets called as the lease is copied in
order to initialise it.

The problem is that lease_alloc() performs an unnecessary initialisation,
which sets the lease_manager_ops.  Avoid the problem by allocating the
target lease structure using locks_alloc_lock().

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>