devlink: add by-instance dump infra

Most dumpit implementations walk the devlink instances.
This requires careful lock taking and reference dropping.
Factor the loop out and provide just a callback to handle
a single instance dump.

Convert one user as an example, other users converted
in the next change.

Slightly inspired by ethtool netlink code.

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>