NTB: MFV 8c9edf63: Fix zero size or integer overflow in ntb_set_mw

A plain 32 bit integer will overflow for values over 4GiB.

Change the plain integer size to the appropriate size type in
ntb_set_mw. Change the type of the size parameter and two local
variables used for size.

Even if there is no overflow, a size of zero is invalid here.

Authored by: Allen Hubbe
Reported by: Juyoung Jung
Obtained from: Linux (Dual BSD/GPL driver)
Sponsored by: EMC / Isilon Storage Division