mlxsw: spectrum: Track sampling triggers in a hash table

Currently, mlxsw supports a single sampling trigger type (i.e., received
packet). When sampling is configured on an ingress port, the sampling
parameters (e.g., pointer to the psample group) are stored as an
attribute of the port, so that they could be passed to
psample_sample_packet() when a sampled packet is trapped to the CPU.

Subsequent patches are going to add more types of sampling triggers,
making it difficult to maintain the current scheme.

Instead, store all the active sampling triggers with their associated
parameters in a hash table. That way, more trigger types can be easily
added.

The next patch will flip mlxsw to use the hash table instead of the
current scheme.

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