devlink: push shared buffer related code into separate file

Cut out another chunk from leftover.c and put sb related code
into a separate file.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://lore.kernel.org/r/20230828061657.300667-4-jiri@resnulli.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>