NTB: perf: Don't require one more memory window than number of peers

ntb_perf should not require more than one memory window per peer. This
was probably an off-by-one error.

Fixes: 5648e56d ("NTB: ntb_perf: Add full multi-port NTB API support")
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Allen Hubbe <allenbh@gmail.com>
Tested-by: Alexander Fomichev <fomichev.ru@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>