Generic DIM

From: Tal Gilboa and Yamin Fridman

Implement net DIM over a generic DIM library, add RDMA DIM

dim.h lib exposes an implementation of the DIM algorithm for
dynamically-tuned interrupt moderation for networking interfaces.

We want a similar functionality for other protocols, which might need to
optimize interrupts differently. Main motivation here is DIM for NVMf
storage protocol.

Current DIM implementation prioritizes reducing interrupt overhead over
latency. Also, in order to reduce DIM's own overhead, the algorithm might
take some time to identify it needs to change profiles. While this is
acceptable for networking, it might not work well on other scenarios.

Here we propose a new structure to DIM. The idea is to allow a slightly
modified functionality without the risk of breaking Net DIM behavior for
netdev. We verified there are no degradations in current DIM behavior with
the modified solution.

Suggested solution:
- Common logic is implemented in lib/dim/dim.c
- Net DIM (existing) logic is implemented in lib/dim/net_dim.c, which uses
  the common logic in dim.c
- Any new DIM logic will be implemented in "lib/dim/new_dim.c".
  This new implementation will expose modified versions of profiles,
  dim_step() and dim_decision().
- DIM API is declared in include/linux/dim.h for all implementations.

Pros for this solution are:
- Zero impact on existing net_dim implementation and usage
- Relatively more code reuse (compared to two separate solutions)
- Increased extensibility

Required for dependencies in the next series.
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>