net/rds: new extension header: rdma bytes

net/rds: new extension header: rdma bytes

Introduce a new extension header type RDSV3_EXTHDR_RDMA_BYTES for
an RDMA initiator to exchange rdma byte counts to its target.
Currently, RDMA operations cannot precisely account how many bytes a
peer just transferred via RDMA, which limits per-connection statistics
and future policy (e.g., monitoring or rate/cgroup accounting of RDMA
traffic).

In this patch we expand rds_message_add_extension to accept multiple
extensions, and add new flag to RDS header: RDS_FLAG_EXTHDR_EXTENSION,
along with a new extension to RDS header: rds_ext_header_rdma_bytes.

Signed-off-by: Shamir Rabinovitch <shamir.rabinovitch@oracle.com>
Signed-off-by: Guangyu Sun <guangyu.sun@oracle.com>
Signed-off-by: Allison Henderson <allison.henderson@oracle.com>
Link: https://patch.msgid.link/20260203055723.1085751-2-achender@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>