ntb: Add error path/handling to Debug FS entry creation

If a failure occurs when creating Debug FS entries, unroll all of
the work that's been done.

Signed-off-by: Gary R Hook <gary.hook@amd.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>