selftests: forwarding: devlink_lib: Add shared buffer helpers

Add helpers to obtain, set, and restore a pool size, and a port-pool and
tc-pool threshold.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>