selftests: mirror: mirror_test(): Allow exact count of packets

The mirroring selftests work by sending ICMP traffic between two hosts.
Along the way, this traffic is mirrored to a gretap netdevice, and counter
taps are then installed strategically along the path of the mirrored
traffic to verify the mirroring took place.

The problem with this is that besides mirroring the primary traffic, any
other service traffic is mirrored as well. At the same time, because the
tests need to work in HW-offloaded scenarios, the ability of the device to
do arbitrary packet inspection should not be taken for granted. Most tests
therefore simply use matchall, one uses flower to match on IP address.

As a result, the selftests are noisy, because besides the primary ICMP
traffic, any amount of other service traffic is mirrored as well.

mirror_test() accommodated this noisiness by giving the counters an
allowance of several packets. But in the previous patch, where possible,
counter taps were changed to match only on an exact ICMP message. At least
in those cases, we can demand an exact number of packets to match.

Where the tap is installed on a connective netdevice, the exact matching is
not practical (though with u32, anything is possible). In those places,
there should still be some leeway -- and probably bigger than before,
because experience shows that these tests are very noisy.

To that end, change mirror_test() so that it can be either called with an
exact number to expect, or with an expression. Where leeway is needed,
adjust callers to pass a ">= 10" instead of mere 10.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>