net/smc: {at|de}tach sndbuf to peer DMB if supported

If the device used by SMC-D supports merging local sndbuf to peer DMB,
then create sndbuf descriptor and attach it to peer DMB once peer
token is obtained, and detach and free the sndbuf descriptor when the
connection is freed.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
Reviewed-by: Wenjia Zhang <wenjia@linux.ibm.com>
Reviewed-and-tested-by: Jan Karcher <jaka@linux.ibm.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>