DLM: increase socket backlog to avoid hangs with 16 nodes

On a 16 node virtual cluster with e1000 NICs joining the 12th node prints
SYN flood warnings for the DLM port:
Dec 21 01:46:41 localhost kernel: [ 2146.516664] TCP: request_sock_TCP: Possible SYN flooding on port 21064. Sending cookies.  Check SNMP counters.

And then joining a DLM lockspace hangs:
```
Dec 21 01:49:00 localhost kernel: [ 2285.780913] INFO: task xapi-clusterd:17638 blocked for more than 120 seconds.
Dec 21 01:49:00 localhost kernel: [ 2285.786476]       Not tainted 4.4.0+10 #1
Dec 21 01:49:00 localhost kernel: [ 2285.789043] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Dec 21 01:49:00 localhost kernel: [ 2285.794611] xapi-clusterd   D ffff88001930bc58     0 17638      1 0x00000000
Dec 21 01:49:00 localhost kernel: [ 2285.794615]  ffff88001930bc58 ffff880025593800 ffff880022433800 ffff88001930c000
Dec 21 01:49:00 localhost kernel: [ 2285.794617]  ffff88000ef4a660 ffff88000ef4a658 ffff880022433800 ffff88000ef4a000
Dec 21 01:49:00 localhost kernel: [ 2285.794619]  ffff88001930bc70 ffffffff8159f6b4 7fffffffffffffff ffff88001930bd10
Dec 21 01:49:00 localhost kernel: [ 2285.794644]  [<ffffffff811570fe>] ? printk+0x4d/0x4f
Dec 21 01:49:00 localhost kernel: [ 2285.794647]  [<ffffffff810b1741>] ? __raw_callee_save___pv_queued_spin_unlock+0x11/0x20
Dec 21 01:49:00 localhost kernel: [ 2285.794649]  [<ffffffff815a085d>] wait_for_completion+0x9d/0x110
Dec 21 01:49:00 localhost kernel: [ 2285.794653]  [<ffffffff810979e0>] ? wake_up_q+0x80/0x80
Dec 21 01:49:00 localhost kernel: [ 2285.794661]  [<ffffffffa03fa4b8>] dlm_new_lockspace+0x908/0xac0 [dlm]
Dec 21 01:49:00 localhost kernel: [ 2285.794665]  [<ffffffff810aaa60>] ? prepare_to_wait_event+0x100/0x100
Dec 21 01:49:00 localhost kernel: [ 2285.794670]  [<ffffffffa0402e37>] device_write+0x497/0x6b0 [dlm]
Dec 21 01:49:00 localhost kernel: [ 2285.794673]  [<ffffffff811834f0>] ? handle_mm_fault+0x7f0/0x13b0
Dec 21 01:49:00 localhost kernel: [ 2285.794677]  [<ffffffff811b4438>] __vfs_write+0x28/0xd0
Dec 21 01:49:00 localhost kernel: [ 2285.794679]  [<ffffffff811b4b7f>] ? rw_verify_area+0x6f/0xd0
Dec 21 01:49:00 localhost kernel: [ 2285.794681]  [<ffffffff811b4dc1>] vfs_write+0xb1/0x190
Dec 21 01:49:00 localhost kernel: [ 2285.794686]  [<ffffffff8105ffc2>] ? __do_page_fault+0x302/0x420
Dec 21 01:49:00 localhost kernel: [ 2285.794688]  [<ffffffff811b5986>] SyS_write+0x46/0xa0
Dec 21 01:49:00 localhost kernel: [ 2285.794690]  [<ffffffff815a31ae>] entry_SYSCALL_64_fastpath+0x12/0x71
```

The previous limit of 5 seems like an arbitrary number, that doesn't match any
known DLM cluster size upper bound limit.
Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Cc: Christine Caulfield <ccaulfie@redhat.com>
Cc: David Teigland <teigland@redhat.com>
Cc: cluster-devel@redhat.com
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>

fs/dlm/lowcomms.c

@@ -1814,7 +1814,7 @@ static int dlm_listen_for_all(void)
 	sock->sk->sk_data_ready = lowcomms_listen_data_ready;
 	release_sock(sock->sk);
 
-	result = sock->ops->listen(sock, 5);
+	result = sock->ops->listen(sock, 128);
 	if (result < 0) {
 		dlm_close_sock(&listen_con.sock);
 		return result;