net/rds Add getsockopt support for SO_RDS_TRANSPORT

The currently attached transport for a PF_RDS socket may be obtained
from user space by invoking getsockopt(2) using the SO_RDS_TRANSPORT
option at the SOL_RDS level. The integer optval returned will be one
of the RDS_TRANS_* constants defined in linux/rds.h.

Signed-off-by: Sowmini Varadhan <sowmini.varadhan@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>