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

[ Upstream commit ae2be35c ]

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>
Stable-dep-of: 0541db8e ("net/smc: initialize close_work early to avoid warning")
Signed-off-by: Sasha Levin <sashal@kernel.org>