Revert runsc to use RecvMMsg packet dispatcher.

PacketMMap mode has issues due to a kernel bug. This change
reverts us to using recvmmsg instead of a shared ring buffer to
dispatch inbound packets. This will reduce performance but should
be more stable under heavy load till PacketMMap is updated to
use TPacketv3.

See #210 for details.

Perf difference between recvmmsg vs packetmmap.

RecvMMsg :
iperf3 -c 172.17.0.2
Connecting to host 172.17.0.2, port 5201
[  4] local 172.17.0.1 port 43478 connected to 172.17.0.2 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec   778 MBytes  6.53 Gbits/sec  4349    188 KBytes
[  4]   1.00-2.00   sec   786 MBytes  6.59 Gbits/sec  4395    212 KBytes
[  4]   2.00-3.00   sec   756 MBytes  6.34 Gbits/sec  3655    161 KBytes
[  4]   3.00-4.00   sec   782 MBytes  6.56 Gbits/sec  4419    175 KBytes
[  4]   4.00-5.00   sec   755 MBytes  6.34 Gbits/sec  4317    187 KBytes
[  4]   5.00-6.00   sec   774 MBytes  6.49 Gbits/sec  4002    173 KBytes
[  4]   6.00-7.00   sec   737 MBytes  6.18 Gbits/sec  3904    191 KBytes
[  4]   7.00-8.00   sec   530 MBytes  4.44 Gbits/sec  3318    189 KBytes
[  4]   8.00-9.00   sec   487 MBytes  4.09 Gbits/sec  2627    188 KBytes
[  4]   9.00-10.00  sec   770 MBytes  6.46 Gbits/sec  4221    170 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  6.99 GBytes  6.00 Gbits/sec  39207             sender
[  4]   0.00-10.00  sec  6.99 GBytes  6.00 Gbits/sec                  receiver

iperf Done.

PacketMMap:

bhaskerh@gvisor-bench:~/tensorflow$ iperf3 -c 172.17.0.2
Connecting to host 172.17.0.2, port 5201
[  4] local 172.17.0.1 port 43496 connected to 172.17.0.2 port 5201
[ ID] Interval           Transfer     Bandwidth       Retr  Cwnd
[  4]   0.00-1.00   sec   657 MBytes  5.51 Gbits/sec    0   1.01 MBytes
[  4]   1.00-2.00   sec  1021 MBytes  8.56 Gbits/sec    0   1.01 MBytes
[  4]   2.00-3.00   sec  1.21 GBytes  10.4 Gbits/sec   45   1.01 MBytes
[  4]   3.00-4.00   sec  1018 MBytes  8.54 Gbits/sec   15   1.01 MBytes
[  4]   4.00-5.00   sec  1.28 GBytes  11.0 Gbits/sec   45   1.01 MBytes
[  4]   5.00-6.00   sec  1.38 GBytes  11.9 Gbits/sec    0   1.01 MBytes
[  4]   6.00-7.00   sec  1.34 GBytes  11.5 Gbits/sec   45    856 KBytes
[  4]   7.00-8.00   sec  1.23 GBytes  10.5 Gbits/sec    0    901 KBytes
[  4]   8.00-9.00   sec  1010 MBytes  8.48 Gbits/sec    0    923 KBytes
[  4]   9.00-10.00  sec  1.39 GBytes  11.9 Gbits/sec    0    960 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bandwidth       Retr
[  4]   0.00-10.00  sec  11.4 GBytes  9.83 Gbits/sec  150             sender
[  4]   0.00-10.00  sec  11.4 GBytes  9.83 Gbits/sec                  receiver

Updates #210

PiperOrigin-RevId: 244968438
Change-Id: Id461b5cbff2dea6fa55cfc108ea246d8f83da20b

runsc/boot/network.go

@@ -140,7 +140,7 @@ func (n *Network) CreateLinksAndRoutes(args *CreateLinksAndRoutesArgs, _ *struct
 			MTU:                uint32(link.MTU),
 			EthernetHeader:     true,
 			Address:            mac,
-			PacketDispatchMode: fdbased.PacketMMap,
+			PacketDispatchMode: fdbased.RecvMMsg,
 			GSOMaxSize:         link.GSOMaxSize,
 			RXChecksumOffload:  true,
 		})