net: rds: add per rds connection cache statistics

The variable cache_allocs is to indicate how many frags (KiB) are in one
rds connection frag cache.
The command "rds-info -Iv" will output the rds connection cache
statistics as below:
"
RDS IB Connections:
      LocalAddr RemoteAddr Tos SL  LocalDev            RemoteDev
      1.1.1.14 1.1.1.14   58 255  fe80::2:c903:a:7a31 fe80::2:c903:a:7a31
      send_wr=256, recv_wr=1024, send_sge=8, rdma_mr_max=4096,
      rdma_mr_size=257, cache_allocs=12
"
This means that there are about 12KiB frag in this rds connection frag
cache.
Since rds.h in rds-tools is not related with the kernel rds.h, the change
in kernel rds.h does not affect rds-tools.
rds-info in rds-tools 2.0.5 and 2.0.6 is tested with this commit. It works
well.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>