drm/amdkfd: Fix memory leak in kfd_mem_dmamap_userptr()

In the Linux kernel, the following vulnerability has been resolved:

drm/amdkfd: Fix memory leak in kfd_mem_dmamap_userptr()

If the number of pages from the userptr BO differs from the SG BO then the

allocated memory for the SG table doesn't get freed before returning

-EINVAL, which may lead to a memory leak in some error paths. Fix this by

checking the number of pages before allocating memory for the SG table.