net: bridge: fix pskb_may_pull

For trill frames, if the frame is decapsulated the returned frame will
have a skb->data that point to the network part of the inner header
and thus the inner ether header must also be pulled.
This patch add this inner ether header in the pskb_may_pull check

Signed-off-by: Ahmed Amamou <ahmed@gandi.net>
Signed-off-by: William Dauchy <william@gandi.net>