drm/amdkfd: set "r = 0" explicitly before goto

To silence the following Smatch static checker warning:

drivers/gpu/drm/amd/amdgpu/../amdkfd/kfd_svm.c:2615
svm_range_restore_pages()
	warn: missing error code here? 'get_task_mm()' failed. 'r' = '0'

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>