ntb_perf: Wait for link before running test

Instead of returning immediately with an error when the link is
down, wait for the link to come up (or the user sends a SIGINT).

This is to make scripting ntb_perf easier.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>