RDS: TCP: Fix non static symbol warnings

Fixes the following sparse warnings:

net/rds/tcp.c:59:5: warning:
 symbol 'rds_tcp_min_sndbuf' was not declared. Should it be static?
net/rds/tcp.c:60:5: warning:
 symbol 'rds_tcp_min_rcvbuf' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>