Revert "netvsc: optimize calculation of number of slots"

The logic for computing page buffer scatter does not take into
account the impact of compound pages. Therefore the optimization
to compute number of slots was incorrect and could cause stack
corruption a skb was sent with lots of fragments from huge pages.

This reverts commit 60b86665.

Fixes: 60b86665 ("netvsc: optimize calculation of number of slots")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>