if_ntb: Don't roundup MW size to full BAR size unnecessarily

Note that the MW allocation still must be BAR *aligned*. So, this only
loosens the constraints on MW allocation slightly. BAR-aligned does not
play well with large (GB+) BAR sizes.

Going forward, if anyone cares about if_ntb on very large BARs, I
suggest they add functionality to allocate a smaller window than the BAR
size, and set the BAR range to cover a window much larger than the
allocated window. This will require negotiating a window offset and
limit for protocol traffic. None of this is implemented in this
revision.

Sponsored by: EMC / Isilon Storage Division