[quagga]: Increase netlink receive buffer for zebra

[pavel-shirshov]

Otherwise we see following messages from zebra sometimes
zebra[60]: netlink-listen recvmsg overrun: No buffer space available

- What I did
Increased netlink socket receive buffer size for zebra. Otherwise we receive following messages sometimes:
zebra[60]: netlink-listen recvmsg overrun: No buffer space available

- How I did it
Used -s command line parameter of zebra

- How to verify it
Build an image and run it. Currently I didn't find how we can check receive buffer size from the command line. I'll update the pr if I find the way.
- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

201911 branch is using frr, you need to generate a new pr.

[jleveque]

@pavel-shirshov: Can you briefly explain how you settled on this size?

[yxieca]

@jleveque expand Prince's discussion :-)

Basically Pavel set it to max value sysctrl allows.

[pavel-shirshov]

@jleveque By default all sockets are created with default receive buffer equal to 212992
net.core.rmem_default = 212992.
This value definitely is not enough for quagga, because we're keep seeing issues with it here and there. I don't know what is the optimal value for the buffer.
I made it 2Mb here #767
So in this patch I increased quagga value to the maximum.

Probably we should increase default receive buffer for higher values.