netdevsim: Only use sampling truncation length when valid

When the sampling truncation length is invalid (zero), pass the length
of the packet. Without the fix, no payload is reported to user space
when the truncation length is zero.

Fixes: a8700c3dd0a4 ("netdevsim: Add dummy psample implementation")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

netdevsim: Add dummy psample implementation

Allow netdevsim to report "sampled" packets to the psample module by
periodically generating packets from a work queue. The behavior can be
enabled / disabled (default) and the various meta data attributes can be
controlled via debugfs knobs.

This implementation enables both testing of the psample module with all
the optional attributes as well as development of user space
applications on top of psample such as hsflowd and a Wireshark dissector
for psample generic netlink packets.

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