ntb_pingpong: Add a debugfs file to get the ping count

This commit adds a debugfs 'count' file to ntb_pingpong. This is so
testing with ntb_pingpong can be automated beyond just checking the
logs for pong messages.

The count file returns a number which increments every pong. The
counter can be cleared by writing a zero.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Allen Hubbe <Allen.Hubbe@emc.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>