Limit memory window usage in ntb_transport to 256MB.

This adds safety net for the case of misconfigured NTB with too big
memory window, for which we may be unable to allocate a memory buffer,
which does not make much sense for the network interface. While there,
fix the code to really work with asymmetric window sizes setup.

This makes driver just print warning message on boot instead of hanging
if too large memory window is configured.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.