dibs: Define dibs loopback

dibs: Define dibs loopback

The first stage of loopback-ism was implemented as part of the
SMC module [1]. Now that we have the dibs layer, provide access to a
dibs_loopback device to all dibs clients.

This is the first step of moving loopback-ism from net/smc/smc_loopback.*
to drivers/dibs/dibs_loopback.*. One global structure lo_dev is allocated
and added to the dibs devices. Follow-on patches will move functionality.

Same as smc_loopback, dibs_loopback is provided by a config option.
Note that there is no way to dynamically add or remove the loopback
device. That could be a future improvement.

When moving code to drivers/dibs, replace ism_ prefix with dibs_ prefix.
As this is mostly a move of existing code, copyright and authors are
unchanged.

Link: https://lore.kernel.org/lkml/20240428060738.60843-1-guwen@linux.alibaba.com/
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Link: https://patch.msgid.link/20250918110500.1731261-7-wintera@linux.ibm.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>