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: a8700c3d ("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>