RDS: make m_rdma_op a member of rds_message

This eliminates a separate memory alloc, although
it is now necessary to add an "r_active" flag, since
it is no longer to use the m_rdma_op pointer as an
indicator of if an rdma op is present.

rdma SGs allocated from rm sg pool.

rds_rm_size also gets bigger. It's a little inefficient to
run through CMSGs twice, but it makes later steps a lot smoother.

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

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

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

diff --cc net/rds/message.c
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