bna: fix skb->truesize underestimation

bna: fix skb->truesize underestimation

skb->truesize is not meant to be tracking amount of used bytes
in an skb, but amount of reserved/consumed bytes in memory.

For instance, if we use a single byte in last page fragment,
we have to account the full size of the fragment.

skb->truesize can be very different from skb->len, that has
a very specific safety purpose.

Signed-off-by: Eric Dumazet <[email protected]>
Cc: Rasesh Mody <[email protected]>
Signed-off-by: David S. Miller <[email protected]>