Currently, the hv_sock buffer size is static and can't scale to the
bandwidth requirements of the application. This change allows the
applications to influence the socket buffer sizes using the SO_SNDBUF and
the SO_RCVBUF socket options.

Few interesting points to note:
1. Since the VMBUS does not allow a resize operation of the ring size, the
socket buffer size option should be set prior to establishing the
connection for it to take effect.
2. Setting the socket option comes with the cost of that much memory being
reserved/allocated by the kernel, for the lifetime of the connection.

Perf data:
Total Data Transfer: 1GB
Single threaded reader/writer
Results below are summarized over 10 iterations.

Linux hvsocket writer + Windows hvsocket reader:
|---------------------------------------------------------------------------------------------|
|Packet size ->   |      128B       |       1KB       |       4KB       |        64KB         |
|---------------------------------------------------------------------------------------------|
|SO_SNDBUF size | |                 Throughput in MB/s (min/max/avg/median):                  |
|               v |                                                                           |
|---------------------------------------------------------------------------------------------|
|      Default    | 109/118/114/116 | 636/774/701/700 | 435/507/480/476 |   410/491/462/470   |
|      16KB       | 110/116/112/111 | 575/705/662/671 | 749/900/854/869 |   592/824/692/676   |
|      32KB       | 108/120/115/115 | 703/823/767/772 | 718/878/850/866 | 1593/2124/2000/2085 |
|      64KB       | 108/119/114/114 | 592/732/683/688 | 805/934/903/911 | 1784/1943/1862/1843 |
|---------------------------------------------------------------------------------------------|

Windows hvsocket writer + Linux hvsocket reader:
|---------------------------------------------------------------------------------------------|
|Packet size ->   |     128B    |      1KB        |          4KB        |        64KB         |
|---------------------------------------------------------------------------------------------|
|SO_RCVBUF size | |               Throughput in MB/s (min/max/avg/median):                    |
|               v |                                                                           |
|---------------------------------------------------------------------------------------------|
|      Default    | 69/82/75/73 | 313/343/333/336 |   418/477/446/445   |   659/701/676/678   |
|      16KB       | 69/83/76/77 | 350/401/375/382 |   506/548/517/516   |   602/624/615/615   |
|      32KB       | 62/83/73/73 | 471/529/496/494 |   830/1046/935/939  | 944/1180/1070/1100  |
|      64KB       | 64/70/68/69 | 467/533/501/497 | 1260/1590/1430/1431 | 1605/1819/1670/1660 |
|---------------------------------------------------------------------------------------------|
Signed-off-by: Sunil Muthuswamy <sunilmut@microsoft.com>
Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>