RDS: Add flag for silent ops. Do atomic op before RDMA

Add a flag to the API so users can indicate they want
silent operations. This is needed because silent ops
cannot be used with USE_ONCE MRs, so we can't just
assume silent.

Also, change send_xmit to do atomic op before rdma op if
both are present, and centralize the hairy logic to determine if
we want to attempt silent, or not.

Signed-off-by: Andy Grover <andy.grover@oracle.com>

diff --cc include/linux/rds.h
Simple merge

diff --cc net/rds/rdma.c
Simple merge

diff --cc net/rds/rds.h
Simple merge

diff --cc net/rds/send.c
Simple merge