linux/dim: Add completions count to dim_sample

Added a measurement of completions per/msec to allow for completion based
dim algorithms.

In order to use dynamic interrupt moderation with RDMA we need to have a
different measurment than packets per second. This change is meant to
prepare for adding a new DIM method.

All drivers that use net_dim and thus do not need a completion count will
have the completions set to 0.

Signed-off-by: Yamin Friedman <yaminf@mellanox.com>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>