[AF_PACKET]: Fix BPF handling.

This fixes a bug introduced by:

commit fda9ef5d
Author: Dmitry Mishin <dim@openvz.org>
Date:   Thu Aug 31 15:28:39 2006 -0700

    [NET]: Fix sk->sk_filter field access

sk_run_filter() returns either 0 or an unsigned 32-bit
length which says how much of the packet to retain.
If that 32-bit unsigned integer is larger than the packet,
this is fine we just leave the packet unchanged.

The above commit caused all filter return values which
were negative when interpreted as a signed integer to
indicate a packet drop, which is wrong.

Based upon a report and initial patch by Raivis Bucis.

Signed-off-by: David S. Miller <davem@davemloft.net>