packet: fix a couple of cppcheck warnings

Doesn't bring much, but also doesn't hurt us to fix 'em:

1) In tpacket_rcv() flush dcache page we can restirct the scope
   for start and end and remove one layer of indent.

2) In tpacket_destruct_skb() we can restirct the scope for ph.

3) In alloc_one_pg_vec_page() we can remove the NULL assignment
   and change spacing a bit.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>