selftests: forwarding: tc_rule_stats_get: Parameterize direction

The GRE multipath tests need stats on an egress counter. Change
tc_rule_stats_get() to take direction as an optional argument, with
default of ingress.

Take the opportunity to change line continuation character from | to \.
Move the | to the next line, which indent.

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