Bug-fix: avoid free NULL pointer in RMA.
req->dev.user_buf points to the data sent from origin process to target process, and for FOP sometimes it points to the IMMED area in packet header when data can be fit in packet header. In such case, we should not free req->dev.user_buf in final request handler since that data area will be freed by the runtime when packet header is freed. In this patch we initialize user_buf to NULL when creating the request, and set it to NULL when FOP is completed, and avoid free a NULL pointer in final request handler. Signed-off-by: Min Si <firstname.lastname@example.org>