nouveau/svm: Use kvcalloc() instead of kvzalloc()

Use 2-factor argument form kvcalloc() instead of kvzalloc().

Link: https://github.com/KSPP/linux/issues/162
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>