esp: Fix skb tailroom calculation

esp: Fix skb tailroom calculation

We use skb_availroom to calculate the skb tailroom for the
ESP trailer. skb_availroom calculates the tailroom and
subtracts this value by reserved_tailroom. However
reserved_tailroom is a union with the skb mark. This means
that we subtract the tailroom by the skb mark if set.
Fix this by using skb_tailroom instead.

Fixes: 1f6f9496b0f0 ("esp4: Avoid skb_cow_data whenever possible")
Fixes: 73a7b070a864 ("esp6: Avoid skb_cow_data whenever possible")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>