For all pages except possibly the last one, the byte beyond the buffer
end must be page aligned.  Therefore, when computing the page shift to
use, OR the end addresses of the buffers as well as the start
addresses into the mask we check.
Signed-off-by: NMichael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: NRoland Dreier <rolandd@cisco.com>