selftests: devlink_lib: Add function for querying maximum pool size

The maximum pool size is exposed via 'devlink sb' command. The next
patch will add a test which increases some pools to the maximum size.

Add a function to query the value.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>