pvcalls-front: fixes incorrect error handling

kfree() is incorrectly used to release the pages allocated by
__get_free_page() and __get_free_pages(). Use the matching deallocators
i.e., free_page() and free_pages(), respectively.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Juergen Gross <jgross@suse.com>