fix the forcedeth NAPI poll function to not emit this warning:

[  186.635916] WARNING: at net/core/dev.c:2166 net_rx_action()
[  186.641351]  [<c060d9f5>] net_rx_action+0x145/0x1b0
[  186.646191]  [<c011d752>] __do_softirq+0x42/0x90
[  186.650784]  [<c011d7c6>] do_softirq+0x26/0x30
[  186.655202]  [<c011db48>] local_bh_enable+0x48/0xa0
[  186.660055]  [<c06023e0>] lock_sock_nested+0xa0/0xc0
[  186.664995]  [<c065da16>] tcp_recvmsg+0x16/0xbc0
[  186.669588]  [<c013e94b>] __generic_file_aio_write_nolock+0x27b/0x520
[  186.676001]  [<c0601d75>] sock_common_recvmsg+0x45/0x70
[  186.681202]  [<c05ff5df>] sock_aio_read+0x11f/0x140
[  186.686054]  [<c015c086>] do_sync_read+0xc6/0x110
[  186.690735]  [<c012b9b0>] autoremove_wake_function+0x0/0x40
[  186.696280]  [<c060dcfc>] net_tx_action+0x3c/0xe0
[  186.700961]  [<c015c9c2>] vfs_read+0x132/0x140
[  186.705378]  [<c015cd41>] sys_read+0x41/0x70
[  186.709625]  [<c0102b66>] sysenter_past_esp+0x5f/0x89
[  186.714651]  =======================

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>