NTB: perf: Fix race condition when run with ntb_test

When running ntb_test, the script tries to run the ntb_perf test
immediately after probing the modules. Since adding multi-port support,
this fails seeing the new initialization procedure in ntb_perf
can not complete instantly.

To fix this we add a completion which is waited on when a test is
started. In this way, run can be written any time after the module is
loaded and it will wait for the initialization to complete instead of
sending an 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>