ntb_perf: Delete duplicate dmaengine_unmap_put() call in perf_copy_chunk()

The function call “dmaengine_unmap_put(unmap)” was used in an if branch.
The same call was immediately triggered by a subsequent goto statement.
Thus avoid such a call repetition.

This issue was detected by using the Coccinelle software.

Fixes: 5648e56d ("NTB: ntb_perf: Add full multi-port NTB API support")
Cc: stable@vger.kernel.org
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Jon Mason <jdmason@kudzu.us>