netfilter: flowtable: validate pppoe header

Ensure there is sufficient room to access the protocol field of the
PPPoe header. Validate it once before the flowtable lookup, then use a
helper function to access protocol field.

Reported-by:  <syzbot+b6f07e1c07ef40199081@syzkaller.appspotmail.com>
Fixes: 72efd585 ("netfilter: flowtable: add pppoe support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>