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 <edumazet@google.com>
Cc: Rasesh Mody <rasesh.mody@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>