selftests: lib: Add a generic helper for obtaining HW stats

The function get_l3_stats() from the test hw_stats_l3.sh will be useful for
any test that wishes to work with L3 stats. Furthermore, it is easy to
generalize to other HW stats suites (for when such are added). Therefore,
move the code to lib.sh, rewrite it to have the same interface as the other
stats-collecting functions, and generalize to take the name of the HW stats
suite to collect as an argument.

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