devlink: allow to use devlink index as a command handle

devlink: allow to use devlink index as a command handle

Currently devlink instances are addressed bus_name/dev_name tuple.
Allow the newly introduced DEVLINK_ATTR_INDEX to be used as
an alternative handle for all devlink commands.

When DEVLINK_ATTR_INDEX is present in the request, use it for a direct
xarray lookup instead of iterating over all instances comparing
bus_name/dev_name strings.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Link: https://patch.msgid.link/20260312100407.551173-5-jiri@resnulli.us
Signed-off-by: Jakub Kicinski <kuba@kernel.org>