sanlock: write lease with clear flag
    
    Add the flag SANLK_WRITE_CLEAR to sanlock_write_resource()
    which causes the resource lease on disk to be written without
    the standard magic number.  This causes a subsequent
    sanlock_read_resource() to return the error -223/SANLK_LEADER_MAGIC
    indicating that no valid lease exists at that location.
    
    This can be used by an application to indicate that the lease area
    on disk is available to be reused for a new resource lease.  (An
    application can continue to use a special resource name of its own
    choosing to implement the same effect.)